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Storage Administration Guide


Red Hat Enterprise Linux 7

Deploying and configuring single-node storage in RHEL 7

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Edited by

Marek Suchánek

Red Hat Customer Content Services
Edited by

Apurva Bhide

Red Hat Customer Content Services

Milan Navrátil

Red Hat Customer Content Services

Jacquelynn East

Red Hat Customer Content Services

Don Domingo

Red Hat Customer Content Services

Abstract

This guide provides instructions on how to effectively manage storage devices and file systems on Red Hat Enterprise Linux 7. It is intended for use by system administrators with basic to intermediate knowledge of Red Hat Enterprise Linux or Fedora.

Chapter 1. Overview

The Storage Administration Guide contains extensive information on supported file systems and data storage features in Red Hat Enterprise Linux 7. This book is intended as a quick reference for administrators managing single-node (that is, non-clustered) storage solutions.
The Storage Administration Guide is split into the following parts: File Systems, Storage Administration, and Data Deduplication and Compression with VDO.
The File Systems part details the various file systems Red Hat Enterprise Linux 7 supports. It describes them and explains how best to utilize them.
The Storage Administration part details the various tools and storage administration tasks Red Hat Enterprise Linux 7 supports. It describes them and explains how best to utilize them.
The Data Deduplication and Compression with VDO part describes the Virtual Data Optimizer (VDO). It explains how to use VDO to reduce your storage requirements.

1.1. New Features and Enhancements in Red Hat Enterprise Linux 7

Red Hat Enterprise Linux 7 features the following file system enhancements:

eCryptfs not included

As of Red Hat Enterprise Linux 7, eCryptfs is not included. For more information on encrypting file systems, see Red Hat's Security Guide.

System Storage Manager

Red Hat Enterprise Linux 7 includes a new application called System Storage Manager which provides a command-line interface to manage various storage technologies. For more information, see Chapter 16, System Storage Manager (SSM).

XFS Is the Default File System

As of Red Hat Enterprise Linux 7, XFS is the default file system. For more information about the XFS file system, see Chapter 3, The XFS File System.

File System Restructure

Red Hat Enterprise Linux 7 introduces a new file system structure. The directories /bin, /sbin, /lib, and /lib64 are now nested under /usr.

Snapper

Red Hat Enterprise Linux 7 introduces a new tool called Snapper that allows for the easy creation and management of snapshots for LVM and Btrfs. For more information, see Chapter 14, Creating and Maintaining Snapshots with Snapper.

Btrfs (Technology Preview)

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.
Btrfs is a local file system that aims to provide better performance and scalability, including integrated LVM operations. This file system is not fully supported by Red Hat and as such is a technology preview. For more information on Btrfs, see Chapter 6, Btrfs (Technology Preview).

NFSv2 No Longer Supported

As of Red Hat Enterprise Linux 7, NFSv2 is no longer supported.

Part I. File Systems

The File Systems section provides information on the file system structure and maintenance, the Btrfs Technology Preview, and file systems that Red Hat fully supports: ext3, ext4, GFS2, XFS, NFS, and FS-Cache.

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.
For an overview of Red Hat Enterprise Linux file systems and storage limits, see Red Hat Enterprise Linux technology capabilities and limits at Red Hat Knowledgebase.
XFS is the default file system in Red Hat Enterprise Linux 7 and Red Hat, and Red Hat recommends you to use XFS unless you have a strong reason to use another file system. For general information on common file systems and their properties, see the following Red Hat Knowledgebase article: How to Choose your Red Hat Enterprise Linux File System.

Chapter 2. File System Structure and Maintenance

The file system structure is the most basic level of organization in an operating system. The way an operating system interacts with its users, applications, and security model nearly always depends on how the operating system organizes files on storage devices. Providing a common file system structure ensures users and programs can access and write files.
File systems break files down into two logical categories:
Shareable and unsharable files
Shareable files can be accessed locally and by remote hosts. Unsharable files are only available locally.
Variable and static files
Variable files, such as documents, can be changed at any time. Static files, such as binaries, do not change without an action from the system administrator.
Categorizing files in this manner helps correlate the function of each file with the permissions assigned to the directories which hold them. How the operating system and its users interact with a file determines the directory in which it is placed, whether that directory is mounted with read-only or read and write permissions, and the level of access each user has to that file. The top level of this organization is crucial; access to the underlying directories can be restricted, otherwise security problems could arise if, from the top level down, access rules do not adhere to a rigid structure.

2.1. Overview of Filesystem Hierarchy Standard (FHS)

Red Hat Enterprise Linux uses the Filesystem Hierarchy Standard (FHS) file system structure, which defines the names, locations, and permissions for many file types and directories.
The FHS document is the authoritative reference to any FHS-compliant file system, but the standard leaves many areas undefined or extensible. This section is an overview of the standard and a description of the parts of the file system not covered by the standard.
The two most important elements of FHS compliance are:
  • Compatibility with other FHS-compliant systems
  • The ability to mount a /usr/ partition as read-only. This is crucial, since /usr/ contains common executables and should not be changed by users. In addition, since /usr/ is mounted as read-only, it should be mountable from the CD-ROM drive or from another machine via a read-only NFS mount.

2.1.1. FHS Organization

The directories and files noted here are a small subset of those specified by the FHS document. For the most complete information, see the latest FHS documentation at http://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.pdf; the file-hierarchy(7) man page also provides an overview.

Note

The directories that are available depend on what is installed on any given system. The following lists are only an example of what may be found.
2.1.1.1. Gathering File System Information
df Command
The df command reports the system's disk space usage. Its output looks similar to the following:

Example 2.1. df Command Output

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                       11675568   6272120   4810348  57% / /dev/sda1
	                 100691      9281     86211  10% /boot
none                     322856         0    322856   0% /dev/shm
By default, df shows the partition size in 1 kilobyte blocks and the amount of used and available disk space in kilobytes. To view the information in megabytes and gigabytes, use the command df -h. The -h argument stands for "human-readable" format. The output for df -h looks similar to the following:

Example 2.2. df -h Command Output

Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
                        12G  6.0G  4.6G  57% / /dev/sda1
			99M  9.1M   85M  10% /boot
none 			316M     0  316M   0% /dev/shm

Note

In the given examples, the mounted partition /dev/shm represents the system's virtual memory file system.
du Command
The du command displays the estimated amount of space being used by files in a directory, displaying the disk usage of each subdirectory. The last line in the output of du shows the total disk usage of the directory. To see only the total disk usage of a directory in human-readable format, use du -hs. For more options, see man du.
Gnome System Monitor
To view the system's partitions and disk space usage in a graphical format, use the Gnome System Monitor by clicking on ApplicationsSystem ToolsSystem Monitor or using the command gnome-system-monitor. Select the File Systems tab to view the system's partitions. The following figure illustrates the File Systems tab.
File Systems Tab in GNOME System Monitor

Figure 2.1. File Systems Tab in GNOME System Monitor

2.1.1.2. The /boot/ Directory
The /boot/ directory contains static files required to boot the system, for example, the Linux kernel. These files are essential for the system to boot properly.

Warning

Do not remove the /boot/ directory. Doing so renders the system unbootable.
2.1.1.3. The /dev/ Directory
The /dev/ directory contains device nodes that represent the following device types:
  • devices attached to the system;
  • virtual devices provided by the kernel.
These device nodes are essential for the system to function properly. The udevd daemon creates and removes device nodes in /dev/ as needed.
Devices in the /dev/ directory and subdirectories are defined as either character (providing only a serial stream of input and output, for example, mouse or keyboard) or block (accessible randomly, such as a hard drive or a floppy drive). If GNOME or KDE is installed, some storage devices are automatically detected when connected (such as with USB) or inserted (such as a CD or DVD drive), and a pop-up window displaying the contents appears.
Table 2.1. Examples of Common Files in the /dev Directory
File Description
/dev/hda The master device on the primary IDE channel.
/dev/hdb The slave device on the primary IDE channel.
/dev/tty0 The first virtual console.
/dev/tty1 The second virtual console.
/dev/sda The first device on the primary SCSI or SATA channel.
/dev/lp0 The first parallel port.
A valid block device can be one of two types of entries:
Mapped device
A logical volume in a volume group, for example, /dev/mapper/VolGroup00-LogVol02.
Static device
A traditional storage volume, for example, /dev/sdbX, where sdb is a storage device name and X is the partition number. /dev/sdbX can also be /dev/disk/by-id/WWID, or /dev/disk/by-uuid/UUID. For more information, see Section 25.8, “Persistent Naming”.
2.1.1.4. The /etc/ Directory
The /etc/ directory is reserved for configuration files that are local to the machine. It should not contain any binaries; if there are any binaries, move them to /usr/bin/ or /usr/sbin/.
For example, the /etc/skel/ directory stores "skeleton" user files, which are used to populate a home directory when a user is first created. Applications also store their configuration files in this directory and may reference them when executed. The /etc/exports file controls which file systems export to remote hosts.
2.1.1.5. The /mnt/ Directory
The /mnt/ directory is reserved for temporarily mounted file systems, such as NFS file system mounts. For all removable storage media, use the /media/ directory. Automatically detected removable media is mounted in the /media directory.

Important

The /mnt directory must not be used by installation programs.
2.1.1.6. The /opt/ Directory
The /opt/ directory is normally reserved for software and add-on packages that are not part of the default installation. A package that installs to /opt/ creates a directory bearing its name, for example, /opt/packagename/. In most cases, such packages follow a predictable subdirectory structure; most store their binaries in /opt/packagename/bin/ and their man pages in /opt/packagename/man/.
2.1.1.7. The /proc/ Directory
The /proc/ directory contains special files that either extract information from the kernel or send information to it. Examples of such information include system memory, CPU information, and hardware configuration. For more information about /proc/, see Section 2.3, “The /proc Virtual File System”.
2.1.1.8. The /srv/ Directory
The /srv/ directory contains site-specific data served by a Red Hat Enterprise Linux system. This directory gives users the location of data files for a particular service, such as FTP, WWW, or CVS. Data that only pertains to a specific user should go in the /home/ directory.
2.1.1.9. The /sys/ Directory
The /sys/ directory utilizes the new sysfs virtual file system specific to the kernel. With the increased support for hot plug hardware devices in the kernel, the /sys/ directory contains information similar to that held by /proc/, but displays a hierarchical view of device information specific to hot plug devices.
2.1.1.10. The /usr/ Directory
The /usr/ directory is for files that can be shared across multiple machines. The /usr/ directory is often on its own partition and is mounted read-only. At a minimum, /usr/ should contain the following subdirectories:
/usr/bin
This directory is used for binaries.
/usr/etc
This directory is used for system-wide configuration files.
/usr/games
This directory stores games.
/usr/include
This directory is used for C header files.
/usr/kerberos
This directory is used for Kerberos-related binaries and files.
/usr/lib
This directory is used for object files and libraries that are not designed to be directly utilized by shell scripts or users.
As of Red Hat Enterprise Linux 7.0, the /lib/ directory has been merged with /usr/lib. Now it also contains libraries needed to execute the binaries in /usr/bin/ and /usr/sbin/. These shared library images are used to boot the system or execute commands within the root file system.
/usr/libexec
This directory contains small helper programs called by other programs.
/usr/sbin
As of Red Hat Enterprise Linux 7.0, /sbin has been moved to /usr/sbin. This means that it contains all system administration binaries, including those essential for booting, restoring, recovering, or repairing the system. The binaries in /usr/sbin/ require root privileges to use.
/usr/share
This directory stores files that are not architecture-specific.
/usr/src
This directory stores source code.
/usr/tmp linked to /var/tmp
This directory stores temporary files.
The /usr/ directory should also contain a /local/ subdirectory. As per the FHS, this subdirectory is used by the system administrator when installing software locally, and should be safe from being overwritten during system updates. The /usr/local directory has a structure similar to /usr/, and contains the following subdirectories:
  • /usr/local/bin
  • /usr/local/etc
  • /usr/local/games
  • /usr/local/include
  • /usr/local/lib
  • /usr/local/libexec
  • /usr/local/sbin
  • /usr/local/share
  • /usr/local/src
Red Hat Enterprise Linux's usage of /usr/local/ differs slightly from the FHS. The FHS states that /usr/local/ should be used to store software that should remain safe from system software upgrades. Since the RPM Package Manager can perform software upgrades safely, it is not necessary to protect files by storing them in /usr/local/.
Instead, Red Hat Enterprise Linux uses /usr/local/ for software local to the machine. For instance, if the /usr/ directory is mounted as a read-only NFS share from a remote host, it is still possible to install a package or program under the /usr/local/ directory.
2.1.1.11. The /var/ Directory
Since the FHS requires Linux to mount /usr/ as read-only, any programs that write log files or need spool/ or lock/ directories should write them to the /var/ directory. The FHS states /var/ is for variable data, which includes spool directories and files, logging data, transient and temporary files.
Following are some of the directories found within the /var/ directory:
  • /var/account/
  • /var/arpwatch/
  • /var/cache/
  • /var/crash/
  • /var/db/
  • /var/empty/
  • /var/ftp/
  • /var/gdm/
  • /var/kerberos/
  • /var/lib/
  • /var/local/
  • /var/lock/
  • /var/log/
  • /var/mail linked to /var/spool/mail/
  • /var/mailman/
  • /var/named/
  • /var/nis/
  • /var/opt/
  • /var/preserve/
  • /var/run/
  • /var/spool/
  • /var/tmp/
  • /var/tux/
  • /var/www/
  • /var/yp/

Important

The /var/run/media/user directory contains subdirectories used as mount points for removable media such as USB storage media, DVDs, CD-ROMs, and Zip disks. Note that previously, the /media/ directory was used for this purpose.
System log files, such as messages and lastlog, go in the /var/log/ directory. The /var/lib/rpm/ directory contains RPM system databases. Lock files go in the /var/lock/ directory, usually in directories for the program using the file. The /var/spool/ directory has subdirectories that store data files for some programs. These subdirectories include:
  • /var/spool/at/
  • /var/spool/clientmqueue/
  • /var/spool/cron/
  • /var/spool/cups/
  • /var/spool/exim/
  • /var/spool/lpd/
  • /var/spool/mail/
  • /var/spool/mailman/
  • /var/spool/mqueue/
  • /var/spool/news/
  • /var/spool/postfix/
  • /var/spool/repackage/
  • /var/spool/rwho/
  • /var/spool/samba/
  • /var/spool/squid/
  • /var/spool/squirrelmail/
  • /var/spool/up2date/
  • /var/spool/uucp/
  • /var/spool/uucppublic/
  • /var/spool/vbox/

2.2. Special Red Hat Enterprise Linux File Locations

Red Hat Enterprise Linux extends the FHS structure slightly to accommodate special files.
Most files pertaining to RPM are kept in the /var/lib/rpm/ directory. For more information on RPM, see man rpm.
The /var/cache/yum/ directory contains files used by the Package Updater, including RPM header information for the system. This location may also be used to temporarily store RPMs downloaded while updating the system. For more information about the Red Hat Network, see https://rhn.redhat.com/.
Another location specific to Red Hat Enterprise Linux is the /etc/sysconfig/ directory. This directory stores a variety of configuration information. Many scripts that run at boot time use the files in this directory.

2.3. The /proc Virtual File System

Unlike most file systems, /proc contains neither text nor binary files. Because it houses virtual files, the /proc is referred to as a virtual file system. These virtual files are typically zero bytes in size, even if they contain a large amount of information.
The /proc file system is not used for storage. Its main purpose is to provide a file-based interface to hardware, memory, running processes, and other system components. Real-time information can be retrieved on many system components by viewing the corresponding /proc file. Some of the files within /proc can also be manipulated (by both users and applications) to configure the kernel.
The following /proc files are relevant in managing and monitoring system storage:
/proc/devices
Displays various character and block devices that are currently configured.
/proc/filesystems
Lists all file system types currently supported by the kernel.
/proc/mdstat
Contains current information on multiple-disk or RAID configurations on the system, if they exist.
/proc/mounts
Lists all mounts currently used by the system.
/proc/partitions
Contains partition block allocation information.
For more information about the /proc file system, see the Red Hat Enterprise Linux 7 Deployment Guide.

2.4. Discard Unused Blocks

Batch discard and online discard operations are features of mounted file systems that discard blocks not in use by the file system. They are useful for both solid-state drives and thinly-provisioned storage.
  • Batch discard operations are run explicitly by the user with the fstrim command. This command discards all unused blocks in a file system that match the user's criteria.
  • Online discard operations are specified at mount time, either with the -o discard option as part of a mount command or with the discard option in the /etc/fstab file. They run in real time without user intervention. Online discard operations only discard blocks that are transitioning from used to free.
Both operation types are supported for use with ext4 file systems as of Red Hat Enterprise Linux 6.2 and later and with XFS file systems since Red Hat Enterprise Linux 6.4. Also, the block device underlying the file system must support physical discard operations. Physical discard operations are supported if the value stored in the /sys/block/device/queue/discard_max_bytes file is not zero.
If you are executing the fstrim command on:
  • a device that does not support discard operations, or
  • a logical device (LVM or MD) comprised of multiple devices, where any one of the device does not support discard operations
the following message will be displayed:
fstrim -v /mnt/non_discard
fstrim: /mnt/non_discard: the discard operation is not supported

Note

The mount command allows you to mount a device that does not support discard operations with the -o discard option.
Red Hat recommends batch discard operations unless the system's workload is such that batch discard is not feasible, or online discard operations are necessary to maintain performance.
For more information, see the fstrim(8) and mount(8) man pages.

Chapter 3. The XFS File System

XFS is a highly scalable, high-performance file system which was originally designed at Silicon Graphics, Inc. XFS is the default file system for Red Hat Enterprise Linux 7.
Main Features of XFS
  • XFS supports metadata journaling, which facilitates quicker crash recovery.
  • The XFS file system can be defragmented and enlarged while mounted and active.
  • In addition, Red Hat Enterprise Linux 7 supports backup and restore utilities specific to XFS.
Allocation Features
XFS features the following allocation schemes:
  • Extent-based allocation
  • Stripe-aware allocation policies
  • Delayed allocation
  • Space pre-allocation
Delayed allocation and other performance optimizations affect XFS the same way that they do ext4. Namely, a program's writes to an XFS file system are not guaranteed to be on-disk unless the program issues an fsync() call afterwards.
For more information on the implications of delayed allocation on a file system (ext4 and XFS), see Allocation Features in Chapter 5, The ext4 File System.

Note

Creating or expanding files occasionally fails with an unexpected ENOSPC write failure even though the disk space appears to be sufficient. This is due to XFS's performance-oriented design. In practice, it does not become a problem since it only occurs if remaining space is only a few blocks.
Other XFS Features
The XFS file system also supports the following:
Extended attributes (xattr)
This allows the system to associate several additional name/value pairs per file. It is enabled by default.
Quota journaling
This avoids the need for lengthy quota consistency checks after a crash.
Project/directory quotas
This allows quota restrictions over a directory tree.
Subsecond timestamps
This allows timestamps to go to the subsecond.
Default atime behavior is relatime
Relatime is on by default for XFS. It has almost no overhead compared to noatime while still maintaining sane atime values.

3.1. Creating an XFS File System

  • To create an XFS file system, use the following command:
    # mkfs.xfs block_device
    • Replace block_device with the path to a block device. For example, /dev/sdb1, /dev/disk/by-uuid/05e99ec8-def1-4a5e-8a9d-5945339ceb2a, or /dev/my-volgroup/my-lv.
    • In general, the default options are optimal for common use.
    • When using mkfs.xfs on a block device containing an existing file system, add the -f option to overwrite that file system.

Example 3.1. mkfs.xfs Command Output

Following is a sample output of the mkfs.xfs command:
meta-data=/dev/device            isize=256    agcount=4, agsize=3277258 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=13109032, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal log           bsize=4096   blocks=6400, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Note

After an XFS file system is created, its size cannot be reduced. However, it can still be enlarged using the xfs_growfs command. For more information, see Section 3.4, “Increasing the Size of an XFS File System”).

Striped Block Devices

For striped block devices (for example, RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances the performance of an XFS filesystem.
When creating filesystems on LVM or MD volumes, mkfs.xfs chooses an optimal geometry. This may also be true on some hardware RAIDs that export geometry information to the operating system.
If the device exports stripe geometry information, the mkfs utility (for ext3, ext4, and xfs) will automatically use this geometry. If stripe geometry is not detected by the mkfs utility and even though the storage does, in fact, have stripe geometry, it is possible to manually specify it when creating the file system using the following options:
su=value
Specifies a stripe unit or RAID chunk size. The value must be specified in bytes, with an optional k, m, or g suffix.
sw=value
Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.
The following example specifies a chunk size of 64k on a RAID device containing 4 stripe units:
# mkfs.xfs -d su=64k,sw=4 /dev/block_device

Additional Resources

For more information about creating XFS file systems, see:

3.2. Mounting an XFS File System

An XFS file system can be mounted with no extra options, for example:
# mount /dev/device /mount/point
The default for Red Hat Enterprise Linux 7 is inode64.

Note

Unlike mke2fs, mkfs.xfs does not utilize a configuration file; they are all specified on the command line.

Write Barriers

By default, XFS uses write barriers to ensure file system integrity even when power is lost to a device with write caches enabled. For devices without write caches, or with battery-backed write caches, disable the barriers by using the nobarrier option:
# mount -o nobarrier /dev/device /mount/point
For more information about write barriers, see Chapter 22, Write Barriers.

Direct Access Technology Preview

Since Red Hat Enterprise Linux 7.3, Direct Access (DAX) is available as a Technology Preview on the ext4 and XFS file systems. It is a means for an application to directly map persistent memory into its address space. To use DAX, a system must have some form of persistent memory available, usually in the form of one or more Non-Volatile Dual Inline Memory Modules (NVDIMMs), and a file system that supports DAX must be created on the NVDIMM(s). Also, the file system must be mounted with the dax mount option. Then, an mmap of a file on the dax-mounted file system results in a direct mapping of storage into the application's address space.

3.3. XFS Quota Management

The XFS quota subsystem manages limits on disk space (blocks) and file (inode) usage. XFS quotas control or report on usage of these items on a user, group, or directory or project level. Also, note that while user, group, and directory or project quotas are enabled independently, group and project quotas are mutually exclusive.
When managing on a per-directory or per-project basis, XFS manages the disk usage of directory hierarchies associated with a specific project. In doing so, XFS recognizes cross-organizational "group" boundaries between projects. This provides a level of control that is broader than what is available when managing quotas for users or groups.
XFS quotas are enabled at mount time, with specific mount options. Each mount option can also be specified as noenforce; this allows usage reporting without enforcing any limits. Valid quota mount options are:
  • uquota/uqnoenforce: User quotas
  • gquota/gqnoenforce: Group quotas
  • pquota/pqnoenforce: Project quota
Once quotas are enabled, the xfs_quota tool can be used to set limits and report on disk usage. By default, xfs_quota is run interactively, and in basic mode. Basic mode subcommands simply report usage, and are available to all users. Basic xfs_quota subcommands include:
quota username/userID
Show usage and limits for the given username or numeric userID
df
Shows free and used counts for blocks and inodes.
In contrast, xfs_quota also has an expert mode. The subcommands of this mode allow actual configuration of limits, and are available only to users with elevated privileges. To use expert mode subcommands interactively, use the following command:
# xfs_quota -x
Expert mode subcommands include:
report /path
Reports quota information for a specific file system.
limit
Modify quota limits.
For a complete list of subcommands for either basic or expert mode, use the subcommand help.
All subcommands can also be run directly from a command line using the -c option, with -x for expert subcommands.

Example 3.2. Display a Sample Quota Report

For example, to display a sample quota report for /home (on /dev/blockdevice), use the command xfs_quota -x -c 'report -h' /home. This displays output similar to the following:
User quota on /home (/dev/blockdevice)
Blocks
User ID      Used   Soft   Hard Warn/Grace
---------- ---------------------------------
root            0      0      0  00 [------]
testuser   103.4G      0      0  00 [------]
...
To set a soft and hard inode count limit of 500 and 700 respectively for user john, whose home directory is /home/john, use the following command:
# xfs_quota -x -c 'limit isoft=500 ihard=700 john' /home/
In this case, pass mount_point which is the mounted xfs file system.
By default, the limit subcommand recognizes targets as users. When configuring the limits for a group, use the -g option (as in the previous example). Similarly, use -p for projects.
Soft and hard block limits can also be configured using bsoft or bhard instead of isoft or ihard.

Example 3.3. Set a Soft and Hard Block Limit

For example, to set a soft and hard block limit of 1000m and 1200m, respectively, to group accounting on the /target/path file system, use the following command:
# xfs_quota -x -c 'limit -g bsoft=1000m bhard=1200m accounting' /target/path

Note

The commands bsoft and bhard count by the byte.

Important

While real-time blocks (rtbhard/rtbsoft) are described in man xfs_quota as valid units when setting quotas, the real-time sub-volume is not enabled in this release. As such, the rtbhard and rtbsoft options are not applicable.

Setting Project Limits

With XFS file system, you can set quotas on individual directory hierarchies in the file system that are known as managed trees. Each managed tree is uniquely identified by a project ID and an optional project name.
  1. Add the project-controlled directories to /etc/projects. For example, the following adds the /var/log path with a unique ID of 11 to /etc/projects. Your project ID can be any numerical value mapped to your project.
    # echo 11:/var/log >> /etc/projects
  2. Add project names to /etc/projid to map project IDs to project names. For example, the following associates a project called logfiles with the project ID of 11 as defined in the previous step.
    # echo logfiles:11 >> /etc/projid
  3. Initialize the project directory. For example, the following initializes the project directory /var:
    # xfs_quota -x -c 'project -s logfiles' /var
  4. Configure quotas for projects with initialized directories:
    # xfs_quota -x -c 'limit -p bhard=lg logfiles' /var
Generic quota configuration tools (quota, repquota, and edquota for example) may also be used to manipulate XFS quotas. However, these tools cannot be used with XFS project quotas.

Important

Red Hat recommends the use of xfs_quota over all other available tools.
For more information about setting XFS quotas, see man xfs_quota, man projid(5), and man projects(5).

3.4. Increasing the Size of an XFS File System

An XFS file system may be grown while mounted using the xfs_growfs command:
# xfs_growfs /mount/point -D size
The -D size option grows the file system to the specified size (expressed in file system blocks). Without the -D size option, xfs_growfs will grow the file system to the maximum size supported by the device.
Before growing an XFS file system with -D size, ensure that the underlying block device is of an appropriate size to hold the file system later. Use the appropriate resizing methods for the affected block device.

Note

While XFS file systems can be grown while mounted, their size cannot be reduced at all.
For more information about growing a file system, see man xfs_growfs.

3.5. Repairing an XFS File System

To repair an XFS file system, use xfs_repair:
# xfs_repair /dev/device
The xfs_repair utility is highly scalable and is designed to repair even very large file systems with many inodes efficiently. Unlike other Linux file systems, xfs_repair does not run at boot time, even when an XFS file system was not cleanly unmounted. In the event of an unclean unmount, xfs_repair simply replays the log at mount time, ensuring a consistent file system.

Warning

The xfs_repair utility cannot repair an XFS file system with a dirty log. To clear the log, mount and unmount the XFS file system. If the log is corrupt and cannot be replayed, use the -L option ("force log zeroing") to clear the log, that is, xfs_repair -L /dev/device. Be aware that this may result in further corruption or data loss.
For more information about repairing an XFS file system, see man xfs_repair.

3.6. Suspending an XFS File System

To suspend or resume write activity to a file system, use the following command:
# xfs_freeze mount-point
Suspending write activity allows hardware-based device snapshots to be used to capture the file system in a consistent state.

Note

The xfs_freeze utility is provided by the xfsprogs package, which is only available on x86_64.
To suspend (that is, freeze) an XFS file system, use:
# xfs_freeze -f /mount/point
To unfreeze an XFS file system, use:
# xfs_freeze -u /mount/point
When taking an LVM snapshot, it is not necessary to use xfs_freeze to suspend the file system first. Rather, the LVM management tools will automatically suspend the XFS file system before taking the snapshot.
For more information about freezing and unfreezing an XFS file system, see man xfs_freeze.

3.7. Backing Up and Restoring XFS File Systems

XFS file system backup and restoration involve these utilities:
  • xfsdump for creating the backup
  • xfsrestore for restoring from backup

3.7.1. Features of XFS Backup and Restoration

Backup
You can use the xfsdump utility to:
  • Perform backups to regular file images.
    Only one backup can be written to a regular file.
  • Perform backups to tape drives.
    The xfsdump utility also allows you to write multiple backups to the same tape. A backup can span multiple tapes.
    To back up multiple file systems to a single tape device, simply write the backup to a tape that already contains an XFS backup. This appends the new backup to the previous one. By default, xfsdump never overwrites existing backups.
  • Create incremental backups.
    The xfsdump utility uses dump levels to determine a base backup to which other backups are relative. Numbers from 0 to 9 refer to increasing dump levels. An incremental backup only backs up files that have changed since the last dump of a lower level:
    • To perform a full backup, perform a level 0 dump on the file system.
    • A level 1 dump is the first incremental backup after a full backup. The next incremental backup would be level 2, which only backs up files that have changed since the last level 1 dump; and so on, to a maximum of level 9.
  • Exclude files from a backup using size, subtree, or inode flags to filter them.
Restoration
The xfsrestore utility restores file systems from backups produced by xfsdump. The xfsrestore utility has two modes:
  • The simple mode enables users to restore an entire file system from a level 0 dump. This is the default mode.
  • The cumulative mode enables file system restoration from an incremental backup: that is, level 1 to level 9.
A unique session ID or session label identifies each backup. Restoring a backup from a tape containing multiple backups requires its corresponding session ID or label.
To extract, add, or delete specific files from a backup, enter the xfsrestore interactive mode. The interactive mode provides a set of commands to manipulate the backup files.

3.7.2. Backing Up an XFS File System

This procedure describes how to back up the content of an XFS file system into a file or a tape.

Procedure 3.1. Backing Up an XFS File System

  • Use the following command to back up an XFS file system:
    # xfsdump -l level [-L label] -f backup-destination path-to-xfs-filesystem
    • Replace level with the dump level of your backup. Use 0 to perform a full backup or 1 to 9 to perform consequent incremental backups.
    • Replace backup-destination with the path where you want to store your backup. The destination can be a regular file, a tape drive, or a remote tape device. For example, /backup-files/Data.xfsdump for a file or /dev/st0 for a tape drive.
    • Replace path-to-xfs-filesystem with the mount point of the XFS file system you want to back up. For example, /mnt/data/. The file system must be mounted.
    • When backing up multiple file systems and saving them on a single tape device, add a session label to each backup using the -L label option so that it is easier to identify them when restoring. Replace label with any name for your backup: for example, backup_data.

Example 3.4. Backing up Multiple XFS File Systems

  • To back up the content of XFS file systems mounted on the /boot/ and /data/ directories and save them as files in the /backup-files/ directory:
    # xfsdump -l 0 -f /backup-files/boot.xfsdump /boot
    # xfsdump -l 0 -f /backup-files/data.xfsdump /data
  • To back up multiple file systems on a single tape device, add a session label to each backup using the -L label option:
    # xfsdump -l 0 -L "backup_boot" -f /dev/st0 /boot
    # xfsdump -l 0 -L "backup_data" -f /dev/st0 /data
Additional Resources
  • For more information about backing up XFS file systems, see the xfsdump(8) man page.

3.7.3. Restoring an XFS File System from Backup

This procedure describes how to restore the content of an XFS file system from a file or tape backup.
Prerequisites

Procedure 3.2. Restoring an XFS File System from Backup

  • The command to restore the backup varies depending on whether you are restoring from a full backup or an incremental one, or are restoring multiple backups from a single tape device:
    # xfsrestore [-r] [-S session-id] [-L session-label] [-i]
                 -f backup-location restoration-path
    • Replace backup-location with the location of the backup. This can be a regular file, a tape drive, or a remote tape device. For example, /backup-files/Data.xfsdump for a file or /dev/st0 for a tape drive.
    • Replace restoration-path with the path to the directory where you want to restore the file system. For example, /mnt/data/.
    • To restore a file system from an incremental (level 1 to level 9) backup, add the -r option.
    • To restore a backup from a tape device that contains multiple backups, specify the backup using the -S or -L options.
      The -S lets you choose a backup by its session ID, while the -L lets you choose by the session label. To obtain the session ID and session labels, use the xfsrestore -I command.
      Replace session-id with the session ID of the backup. For example, b74a3586-e52e-4a4a-8775-c3334fa8ea2c. Replace session-label with the session label of the backup. For example, my_backup_session_label.
    • To use xfsrestore interactively, use the -i option.
      The interactive dialog begins after xfsrestore finishes reading the specified device. Available commands in the interactive xfsrestore shell include cd, ls, add, delete, and extract; for a complete list of commands, use the help command.

Example 3.5. Restoring Multiple XFS File Systems

To restore the XFS backup files and save their content into directories under /mnt/:
# xfsrestore -f /backup-files/boot.xfsdump /mnt/boot/
# xfsrestore -f /backup-files/data.xfsdump /mnt/data/
To restore from a tape device containing multiple backups, specify each backup by its session label or session ID:
# xfsrestore -f /dev/st0 -L "backup_boot" /mnt/boot/
# xfsrestore -f /dev/st0 -S "45e9af35-efd2-4244-87bc-4762e476cbab" /mnt/data/
Informational Messages When Restoring a Backup from a Tape
When restoring a backup from a tape with backups from multiple file systems, the xfsrestore utility might issue messages. The messages inform you whether a match of the requested backup has been found when xfsrestore examines each backup on the tape in sequential order. For example:
xfsrestore: preparing drive
xfsrestore: examining media file 0
xfsrestore: inventory session uuid (8590224e-3c93-469c-a311-fc8f23029b2a) does not match the media header's session uuid (7eda9f86-f1e9-4dfd-b1d4-c50467912408)
xfsrestore: examining media file 1
xfsrestore: inventory session uuid (8590224e-3c93-469c-a311-fc8f23029b2a) does not match the media header's session uuid (7eda9f86-f1e9-4dfd-b1d4-c50467912408)
[...]
The informational messages keep appearing until the matching backup is found.
Additional Resources
  • For more information about restoring XFS file systems, see the xfsrestore(8) man page.

3.8. Configuring Error Behavior

When an error occurs during an I/O operation, the XFS driver responds in one of two ways:
  • Continue retries until either:
    • the I/O operation succeeds, or
    • an I/O operation retry count or time limit is exceeded.
  • Consider the error permanent and halt the system.
XFS currently recognizes the following error conditions for which you can configure the desired behavior specifically:
  • EIO: Error while trying to write to the device
  • ENOSPC: No space left on the device
  • ENODEV: Device cannot be found
All other possible error conditions, which do not have specific handlers defined, share a single, global configuration.
You can set the conditions under which XFS deems the errors permanent, both in the maximum number of retries and the maximum time in seconds. XFS stops retrying when any one of the conditions is met.
There is also an option to immediately cancel the retries when unmounting the file system, regardless of any other configuration. This allows the unmount operation to succeed even in case of persistent errors.

3.8.1. Configuration Files for Specific and Undefined Conditions

Configuration files controlling error behavior are located in the /sys/fs/xfs/device/error/ directory.
The /sys/fs/xfs/device/error/metadata/ directory contains subdirectories for each specific error condition:
  • /sys/fs/xfs/device/error/metadata/EIO/ for the EIO error condition
  • /sys/fs/xfs/device/error/metadata/ENODEV/ for the ENODEV error condition
  • /sys/fs/xfs/device/error/metadata/ENOSPC/ for the ENOSPC error condition
Each one then contains the following configuration files:
  • /sys/fs/xfs/device/error/metadata/condition/max_retries: controls the maximum number of times that XFS retries the operation.
  • /sys/fs/xfs/device/error/metadata/condition/retry_timeout_seconds: the time limit in seconds after which XFS will stop retrying the operation
All other possible error conditions, apart from those described in the previous section, share a common configuration in these files:
  • /sys/fs/xfs/device/error/metadata/default/max_retries: controls the maximum number of retries
  • /sys/fs/xfs/device/error/metadata/default/retry_timeout_seconds: controls the time limit for retrying

3.8.2. Setting File System Behavior for Specific and Undefined Conditions

To set the maximum number of retries, write the desired number to the max_retries file.
  • For specific conditions:
    # echo value > /sys/fs/xfs/device/error/metadata/condition/max_retries
  • For undefined conditions:
    # echo value > /sys/fs/xfs/device/error/metadata/default/max_retries
value is a number between -1 and the maximum possible value of int, the C signed integer type. This is 2147483647 on 64-bit Linux.
To set the time limit, write the desired number of seconds to the retry_timeout_seconds file.
  • For specific conditions:
    # echo value > /sys/fs/xfs/device/error/metadata/condition/retry_timeout_seconds
  • For undefined conditions:
    # echo value > /sys/fs/xfs/device/error/metadata/default/retry_timeout_seconds
value is a number between -1 and 86400, which is the number of seconds in a day.
In both the max_retries and retry_timeout_seconds options, -1 means to retry forever and 0 to stop immediately.
device is the name of the device, as found in the /dev/ directory; for example, sda.

Note

The default behavior for a each error condition is dependent on the error context. Some errors, like ENODEV, are considered to be fatal and unrecoverable, regardless of the retry count, so their default value is 0.

3.8.3. Setting Unmount Behavior

If the fail_at_unmount option is set, the file system overrides all other error configurations during unmount, and immediately umnounts the file system without retrying the I/O operation. This allows the unmount operation to succeed even in case of persistent errors.
To set the unmount behavior:
# echo value > /sys/fs/xfs/device/error/fail_at_unmount
value is either 1 or 0:
  • 1 means to cancel retrying immediately if an error is found.
  • 0 means to respect the max_retries and retry_timeout_seconds options.
device is the name of the device, as found in the /dev/ directory; for example, sda.

Important

The fail_at_unmount option has to be set as desired before attempting to unmount the file system. After an unmount operation has started, the configuration files and directories may be unavailable.

3.9. Other XFS File System Utilities

Red Hat Enterprise Linux 7 also features other utilities for managing XFS file systems:
xfs_fsr
Used to defragment mounted XFS file systems. When invoked with no arguments, xfs_fsr defragments all regular files in all mounted XFS file systems. This utility also allows users to suspend a defragmentation at a specified time and resume from where it left off later.
In addition, xfs_fsr also allows the defragmentation of only one file, as in xfs_fsr /path/to/file. Red Hat advises not to periodically defrag an entire file system because XFS avoids fragmentation by default. System wide defragmentation could cause the side effect of fragmentation in free space.
xfs_bmap
Prints the map of disk blocks used by files in an XFS filesystem. This map lists each extent used by a specified file, as well as regions in the file with no corresponding blocks (that is, holes).
xfs_info
Prints XFS file system information.
xfs_admin
Changes the parameters of an XFS file system. The xfs_admin utility can only modify parameters of unmounted devices or file systems.
xfs_copy
Copies the contents of an entire XFS file system to one or more targets in parallel.
The following utilities are also useful in debugging and analyzing XFS file systems:
xfs_metadump
Copies XFS file system metadata to a file. Red Hat only supports using the xfs_metadump utility to copy unmounted file systems or read-only mounted file systems; otherwise, generated dumps could be corrupted or inconsistent.
xfs_mdrestore
Restores an XFS metadump image (generated using xfs_metadump) to a file system image.
xfs_db
Debugs an XFS file system.
For more information about these utilities, see their respective man pages.

3.10. Migrating from ext4 to XFS

Starting with Red Hat Enterprise Linux 7.0, XFS is the default file system instead of ext4. This section highlights the differences when using or administering an XFS file system.
The ext4 file system is still fully supported in Red Hat Enterprise Linux 7 and can be selected at installation. While it is possible to migrate from ext4 to XFS, it is not required.

3.10.1. Differences Between Ext3/4 and XFS

File system repair
Ext3/4 runs e2fsck in userspace at boot time to recover the journal as needed. XFS, by comparison, performs journal recovery in kernelspace at mount time. An fsck.xfs shell script is provided but does not perform any useful action as it is only there to satisfy initscript requirements.
When an XFS file system repair or check is requested, use the xfs_repair command. Use the -n option for a read-only check.
The xfs_repair command will not operate on a file system with a dirty log. To repair such a file system mount and unmount must first be performed to replay the log. If the log is corrupt and cannot be replayed, the -L option can be used to zero out in the log.
For more information on file system repair of XFS file systems, see Section 12.2.2, “XFS”
Metadata error behavior
The ext3/4 file system has configurable behavior when metadata errors are encountered, with the default being to simply continue. When XFS encounters a metadata error that is not recoverable it will shut down the file system and return a EFSCORRUPTED error. The system logs will contain details of the error encountered and will recommend running xfs_repair if necessary.
Quotas
XFS quotas are not a remountable option. The -o quota option must be specified on the initial mount for quotas to be in effect.
While the standard tools in the quota package can perform basic quota administrative tasks (tools such as setquota and repquota), the xfs_quota tool can be used for XFS-specific features, such as Project Quota administration.
The quotacheck command has no effect on an XFS file system. The first time quota accounting is turned on XFS does an automatic quotacheck internally. Because XFS quota metadata is a first-class, journaled metadata object, the quota system will always be consistent until quotas are manually turned off.
File system resize
The XFS file system has no utility to shrink a file system. XFS file systems can be grown online via the xfs_growfs command.
Inode numbers
For file systems larger than 1 TB with 256-byte inodes, or larger than 2 TB with 512-byte inodes, XFS inode numbers might exceed 2^32. Such large inode numbers cause 32-bit stat calls to fail with the EOVERFLOW return value. The described problem might occur when using the default Red Hat Enterprise Linux 7 configuration: non-striped with four allocation groups. A custom configuration, for example file system extension or changing XFS file system parameters, might lead to a different behavior.
Applications usually handle such larger inode numbers correctly. If needed, mount the XFS file system with the -o inode32 parameter to enforce inode numbers below 2^32. Note that using inode32 does not affect inodes that are already allocated with 64-bit numbers.

Important

Do not use the inode32 option unless it is required by a specific environment. The inode32 option changes allocation behavior. As a consequence, the ENOSPC error might occur if no space is available to allocate inodes in the lower disk blocks.
Speculative preallocation
XFS uses speculative preallocation to allocate blocks past EOF as files are written. This avoids file fragmentation due to concurrent streaming write workloads on NFS servers. By default, this preallocation increases with the size of the file and will be apparent in "du" output. If a file with speculative preallocation is not dirtied for five minutes the preallocation will be discarded. If the inode is cycled out of cache before that time, then the preallocation will be discarded when the inode is reclaimed.
If premature ENOSPC problems are seen due to speculative preallocation, a fixed preallocation amount may be specified with the -o allocsize=amount mount option.
Fragmentation-related tools
Fragmentation is rarely a significant issue on XFS file systems due to heuristics and behaviors, such as delayed allocation and speculative preallocation. However, tools exist for measuring file system fragmentation as well as defragmenting file systems. Their use is not encouraged.
The xfs_db frag command attempts to distill all file system allocations into a single fragmentation number, expressed as a percentage. The output of the command requires significant expertise to understand its meaning. For example, a fragmentation factor of 75% means only an average of 4 extents per file. For this reason the output of xfs_db's frag is not considered useful and more careful analysis of any fragmentation problems is recommended.

Warning

The xfs_fsr command may be used to defragment individual files, or all files on a file system. The later is especially not recommended as it may destroy locality of files and may fragment free space.
Commands Used with ext3 and ext4 Compared to XFS
The following table compares common commands used with ext3 and ext4 to their XFS-specific counterparts.
Table 3.1. Common Commands for ext3 and ext4 Compared to XFS
Task ext3/4 XFS
Create a file system mkfs.ext4 or mkfs.ext3 mkfs.xfs
File system check e2fsck xfs_repair
Resizing a file system resize2fs xfs_growfs
Save an image of a file system e2image xfs_metadump and xfs_mdrestore
Label or tune a file system tune2fs xfs_admin
Backup a file system dump and restore xfsdump and xfsrestore
The following table lists generic tools that function on XFS file systems as well, but the XFS versions have more specific functionality and as such are recommended.
Table 3.2. Generic Tools for ext4 and XFS
Task ext4 XFS
Quota quota xfs_quota
File mapping filefrag xfs_bmap
More information on many the listed XFS commands is included in Chapter 3, The XFS File System. You can also consult the manual pages of the listed XFS administration tools for more information.

Chapter 4. The ext3 File System

The ext3 file system is essentially an enhanced version of the ext2 file system. These improvements provide the following advantages:
Availability
After an unexpected power failure or system crash (also called an unclean system shutdown), each mounted ext2 file system on the machine must be checked for consistency by the e2fsck program. This is a time-consuming process that can delay system boot time significantly, especially with large volumes containing a large number of files. During this time, any data on the volumes is unreachable.
It is possible to run fsck -n on a live filesystem. However, it will not make any changes and may give misleading results if partially written metadata is encountered.
If LVM is used in the stack, another option is to take an LVM snapshot of the filesystem and run fsck on it instead.
Finally, there is the option to remount the filesystem as read only. All pending metadata updates (and writes) are then forced to the disk prior to the remount. This ensures the filesystem is in a consistent state, provided there is no previous corruption. It is now possible to run fsck -n.
The journaling provided by the ext3 file system means that this sort of file system check is no longer necessary after an unclean system shutdown. The only time a consistency check occurs using ext3 is in certain rare hardware failure cases, such as hard drive failures. The time to recover an ext3 file system after an unclean system shutdown does not depend on the size of the file system or the number of files; rather, it depends on the size of the journal used to maintain consistency. The default journal size takes about a second to recover, depending on the speed of the hardware.

Note

The only journaling mode in ext3 supported by Red Hat is data=ordered (default).
Data Integrity
The ext3 file system prevents loss of data integrity in the event that an unclean system shutdown occurs. The ext3 file system allows you to choose the type and level of protection that your data receives. With regard to the state of the file system, ext3 volumes are configured to keep a high level of data consistency by default.
Speed
Despite writing some data more than once, ext3 has a higher throughput in most cases than ext2 because ext3's journaling optimizes hard drive head motion. You can choose from three journaling modes to optimize speed, but doing so means trade-offs in regards to data integrity if the system was to fail.

Note

The only journaling mode in ext3 supported by Red Hat is data=ordered (default).
Easy Transition
It is easy to migrate from ext2 to ext3 and gain the benefits of a robust journaling file system without reformatting. For more information on performing this task, see Section 4.2, “Converting to an ext3 File System” .

Note

Red Hat Enterprise Linux 7 provides a unified extN driver. It does this by disabling the ext2 and ext3 configurations and instead uses ext4.ko for these on-disk formats. This means that kernel messages will always refer to ext4 regardless of the ext file system used.

4.1. Creating an ext3 File System

After installation, it is sometimes necessary to create a new ext3 file system. For example, if a new disk drive is added to the system, you may want to partition the drive and use the ext3 file system.
  1. Format the partition or LVM volume with the ext3 file system using the mkfs.ext3 utility:
    # mkfs.ext3 block_device
    • Replace block_device with the path to a block device. For example, /dev/sdb1, /dev/disk/by-uuid/05e99ec8-def1-4a5e-8a9d-5945339ceb2a, or /dev/my-volgroup/my-lv.
  2. Label the file system using the e2label utility:
    # e2label block_device volume_label

Configuring UUID

It is also possible to set a specific UUID for a file system. To specify a UUID when creating a file system, use the -U option:
# mkfs.ext3 -U UUID device
  • Replace UUID with the UUID you want to set: for example, 7cd65de3-e0be-41d9-b66d-96d749c02da7.
  • Replace device with the path to an ext3 file system to have the UUID added to it: for example, /dev/sda8.
To change the UUID of an existing file system, see Section 25.8.3.2, “Modifying Persistent Naming Attributes”

Additional Resources

  • The mkfs.ext3(8) man page
  • The e2label(8) man page

4.2. Converting to an ext3 File System

The tune2fs command converts an ext2 file system to ext3.

Note

To convert ext2 to ext3, always use the e2fsck utility to check your file system before and after using tune2fs. Before trying to convert ext2 to ext3, back up all file systems in case any errors occur.
In addition, Red Hat recommends creating a new ext3 file system and migrating data to it, instead of converting from ext2 to ext3 whenever possible.
To convert an ext2 file system to ext3, log in as root and type the following command in a terminal:
# tune2fs -j block_device
block_device contains the ext2 file system to be converted.
Issue the df command to display mounted file systems.

4.3. Reverting to an Ext2 File System

In order to revert to an ext2 file system, use the following procedure.
For simplicity, the sample commands in this section use the following value for the block device:
/dev/mapper/VolGroup00-LogVol02

Procedure 4.1. Revert from ext3 to ext2

  1. Unmount the partition by logging in as root and typing:
    # umount /dev/mapper/VolGroup00-LogVol02
  2. Change the file system type to ext2 by typing the following command:
    # tune2fs -O ^has_journal /dev/mapper/VolGroup00-LogVol02
  3. Check the partition for errors by typing the following command:
    # e2fsck -y /dev/mapper/VolGroup00-LogVol02
  4. Then mount the partition again as ext2 file system by typing:
    # mount -t ext2 /dev/mapper/VolGroup00-LogVol02 /mount/point
    Replace /mount/point with the mount point of the partition.

    Note

    If a .journal file exists at the root level of the partition, delete it.
To permanently change the partition to ext2, remember to update the /etc/fstab file, otherwise it will revert back after booting.

Chapter 5. The ext4 File System

The ext4 file system is a scalable extension of the ext3 file system. With Red Hat Enterprise Linux 7, it can support a maximum individual file size of 16 terabytes, and file systems to a maximum of 50 terabytes, unlike Red Hat Enterprise Linux 6 which only supported file systems up to 16 terabytes. It also supports an unlimited number of sub-directories (the ext3 file system only supports up to 32,000), though once the link count exceeds 65,000 it resets to 1 and is no longer increased. The bigalloc feature is not currently supported.

Note

As with ext3, an ext4 volume must be umounted in order to perform an fsck. For more information, see Chapter 4, The ext3 File System.
Main Features
The ext4 file system uses extents (as opposed to the traditional block mapping scheme used by ext2 and ext3), which improves performance when using large files and reduces metadata overhead for large files. In addition, ext4 also labels unallocated block groups and inode table sections accordingly, which allows them to be skipped during a file system check. This makes for quicker file system checks, which becomes more beneficial as the file system grows in size.
Allocation Features
The ext4 file system features the following allocation schemes:
  • Persistent pre-allocation
  • Delayed allocation
  • Multi-block allocation
  • Stripe-aware allocation
Because of delayed allocation and other performance optimizations, ext4's behavior of writing files to disk is different from ext3. In ext4, when a program writes to the file system, it is not guaranteed to be on-disk unless the program issues an fsync() call afterwards.
By default, ext3 automatically forces newly created files to disk almost immediately even without fsync(). This behavior hid bugs in programs that did not use fsync() to ensure that written data was on-disk. The ext4 file system, on the other hand, often waits several seconds to write out changes to disk, allowing it to combine and reorder writes for better disk performance than ext3.

Warning

Unlike ext3, the ext4 file system does not force data to disk on transaction commit. As such, it takes longer for buffered writes to be flushed to disk. As with any file system, use data integrity calls such as fsync() to ensure that data is written to permanent storage.
Other ext4 Features
The ext4 file system also supports the following:
  • Extended attributes (xattr) — This allows the system to associate several additional name and value pairs per file.
  • Quota journaling — This avoids the need for lengthy quota consistency checks after a crash.

    Note

    The only supported journaling mode in ext4 is data=ordered (default).
  • Subsecond timestamps — This gives timestamps to the subsecond.

5.1. Creating an ext4 File System

  • To create an ext4 file system, use the following command:
    # mkfs.ext4 block_device
    • Replace block_device with the path to a block device. For example, /dev/sdb1, /dev/disk/by-uuid/05e99ec8-def1-4a5e-8a9d-5945339ceb2a, or /dev/my-volgroup/my-lv.
    • In general, the default options are optimal for most usage scenarios.

Example 5.1. mkfs.ext4 Command Output

Below is a sample output of this command, which displays the resulting file system geometry and features:
~]# mkfs.ext4 /dev/sdb1
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
245280 inodes, 979456 blocks
48972 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=1006632960
30 block groups
32768 blocks per group, 32768 fragments per group
8176 inodes per group
Superblock backups stored on blocks:
 32768, 98304, 163840, 229376, 294912, 819200, 884736

Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

Important

It is possible to use tune2fs to enable certain ext4 features on ext3 file systems. However, using tune2fs in this way has not been fully tested and is therefore not supported in Red Hat Enterprise Linux 7. As a result, Red Hat cannot guarantee consistent performance and predictable behavior for ext3 file systems converted or mounted by using tune2fs.

Striped Block Devices

For striped block devices (for example, RAID5 arrays), the stripe geometry can be specified at the time of file system creation. Using proper stripe geometry greatly enhances the performance of an ext4 file system.
When creating file systems on LVM or MD volumes, mkfs.ext4 chooses an optimal geometry. This may also be true on some hardware RAIDs which export geometry information to the operating system.
To specify stripe geometry, use the -E option of mkfs.ext4 (that is, extended file system options) with the following sub-options:
stride=value
Specifies the RAID chunk size.
stripe-width=value
Specifies the number of data disks in a RAID device, or the number of stripe units in the stripe.
For both sub-options, value must be specified in file system block units. For example, to create a file system with a 64k stride (that is, 16 x 4096) on a 4k-block file system, use the following command:
# mkfs.ext4 -E stride=16,stripe-width=64 /dev/block_device

Configuring UUID

It is also possible to set a specific UUID for a file system. To specify a UUID when creating a file system, use the -U option:
# mkfs.ext4 -U UUID device
  • Replace UUID with the UUID you want to set: for example, 7cd65de3-e0be-41d9-b66d-96d749c02da7.
  • Replace device with the path to an ext4 file system to have the UUID added to it: for example, /dev/sda8.
To change the UUID of an existing file system, see Section 25.8.3.2, “Modifying Persistent Naming Attributes”

Additional Resources

For more information about creating ext4 file systems, see:
  • The mkfs.ext4(8) man page

5.2. Mounting an ext4 File System

An ext4 file system can be mounted with no extra options. For example:
# mount /dev/device /mount/point
The ext4 file system also supports several mount options to influence behavior. For example, the acl parameter enables access control lists, while the user_xattr parameter enables user extended attributes. To enable both options, use their respective parameters with -o, as in:
# mount -o acl,user_xattr /dev/device /mount/point
As with ext3, the option data_err=abort can be used to abort the journal if an error occurs in file data.
# mount -o data_err=abort /dev/device /mount/point
The tune2fs utility also allows administrators to set default mount options in the file system superblock. For more information on this, refer to man tune2fs.

Write Barriers

By default, ext4 uses write barriers to ensure file system integrity even when power is lost to a device with write caches enabled. For devices without write caches, or with battery-backed write caches, disable barriers using the nobarrier option, as in:
# mount -o nobarrier /dev/device /mount/point
For more information about write barriers, refer to Chapter 22, Write Barriers.

Direct Access Technology Preview

Starting with Red Hat Enterprise Linux 7.3, Direct Access (DAX) provides, as a Technology Preview on the ext4 and XFS file systems, a means for an application to directly map persistent memory into its address space. To use DAX, a system must have some form of persistent memory available, usually in the form of one or more Non-Volatile Dual In-line Memory Modules (NVDIMMs), and a file system that supports DAX must be created on the NVDIMM(s). Also, the file system must be mounted with the dax mount option. Then, an mmap of a file on the dax-mounted file system results in a direct mapping of storage into the application's address space.

5.3. Resizing an ext4 File System

Before growing an ext4 file system, ensure that the underlying block device is of an appropriate size to hold the file system later. Use the appropriate resizing methods for the affected block device.
An ext4 file system may be grown while mounted using the resize2fs command:
# resize2fs /mount/device size
The resize2fs command can also decrease the size of an unmounted ext4 file system:
# resize2fs /dev/device size
When resizing an ext4 file system, the resize2fs utility reads the size in units of file system block size, unless a suffix indicating a specific unit is used. The following suffixes indicate specific units:
  • s — 512 byte sectors
  • K — kilobytes
  • M — megabytes
  • G — gigabytes

Note

The size parameter is optional (and often redundant) when expanding. The resize2fs automatically expands to fill all available space of the container, usually a logical volume or partition.
For more information about resizing an ext4 file system, refer to man resize2fs.

5.4. Backing up ext2, ext3, or ext4 File Systems

This procedure describes how to back up the content of an ext4, ext3, or ext2 file system into a file.

Prerequisites

  • If the system has been running for a long time, run the e2fsck utility on the partitions before backup:
    # e2fsck /dev/device

Procedure 5.1. Backing up ext2, ext3, or ext4 File Systems

  1. Back up configuration information, including the content of the /etc/fstab file and the output of the fdisk -l command. This is useful for restoring the partitions.
    To capture this information, run the sosreport or sysreport utilities. For more information about sosreport, see the What is a sosreport and how to create one in Red Hat Enterprise Linux 4.6 and later? Kdowledgebase article.
  2. Depending on the role of the partition:
    • If the partition you are backing up is an operating system partition, boot your system into the rescue mode. See the Booting to Rescue Mode section of the System Administrator's Guide.
    • When backing up a regular, data partition, unmount it.
      Although it is possible to back up a data partition while it is mounted, the results of backing up a mounted data partition can be unpredictable.
      If you need to back up a mounted file system using the dump utility, do so when the file system is not under a heavy load. The more activity is happening on the file system when backing up, the higher the risk of backup corruption is.
  3. Use the dump utility to back up the content of the partitions:
    # dump -0uf backup-file /dev/device
    Replace backup-file with a path to a file where you want the to store the backup. Replace device with the name of the ext4 partition you want to back up. Make sure that you are saving the backup to a directory mounted on a different partition than the partition you are backing up.

    Example 5.2. Backing up Multiple ext4 Partitions

    To back up the content of the /dev/sda1, /dev/sda2, and /dev/sda3 partitions into backup files stored in the /backup-files/ directory, use the following commands:
    # dump -0uf /backup-files/sda1.dump /dev/sda1
    # dump -0uf /backup-files/sda2.dump /dev/sda2
    # dump -0uf /backup-files/sda3.dump /dev/sda3
    To do a remote backup, use the ssh utility or configure a password-less ssh login. For more information on ssh and password-less login, see the Using the ssh Utility and Using Key-based Authentication sections of the System Administrator's Guide.
    For example, when using ssh:

    Example 5.3. Performing a Remote Backup Using ssh

    # dump -0u -f - /dev/device | ssh root@remoteserver.example.com dd of=backup-file
    Note that if using standard redirection, you must pass the -f option separately.

Additional Resources

  • For more information, see the dump(8) man page.

5.5. Restoring ext2, ext3, or ext4 File Systems

This procedure describes how to restore an ext4, ext3, or ext2 file system from a file backup.

Prerequisites

Procedure 5.2. Restoring ext2, ext3, or ext4 File Systems

  1. If you are restoring an operating system partition, boot your system into Rescue Mode. See the Booting to Rescue Mode section of the System Administrator's Guide.
    This step is not required for ordinary data partitions.
  2. Rebuild the partitions you want to restore by using the fdisk or parted utilites.
    If the partitions no longer exist, recreate them. The new partitions must be large enough to contain the restored data. It is important to get the start and end numbers right; these are the starting and ending sector numbers of the partitions obtained from the fdisk utility when backing up.
    For more information on modifying partitions, see Chapter 13, Partitions
  3. Use the mkfs utility to format the destination partition:
    # mkfs.ext4 /dev/device

    Important

    Do not format the partition that stores your backup files.
  4. If you created new partitions, re-label all the partitions so they match their entries in the /etc/fstab file:
    # e2label /dev/device label
  5. Create temporary mount points and mount the partitions on them:
    # mkdir /mnt/device
    # mount -t ext4 /dev/device /mnt/device
  6. Restore the data from backup on the mounted partition:
    # cd /mnt/device
    # restore -rf device-backup-file
    If you want to restore on a remote machine or restore from a backup file that is stored on a remote host, you can use the ssh utility. For more information on ssh, see the Using the ssh Utility section of the System Administrator's Guide.
    Note that you need to configure a password-less login for the following commands. For more information on setting up a password-less ssh login, see the Using Key-based Authentication section of the System Administrator's Guide.
    • To restore a partition on a remote machine from a backup file stored on the same machine:
      # ssh remote-address "cd /mnt/device && cat backup-file | /usr/sbin/restore -r -f -"
    • To restore a partition on a remote machine from a backup file stored on a different remote machine:
      # ssh remote-machine-1 "cd /mnt/device && RSH=/usr/bin/ssh /usr/sbin/restore -rf remote-machine-2:backup-file"
  7. Reboot:
    # systemctl reboot

Example 5.4. Restoring Multiple ext4 Partitions

To restore the /dev/sda1, /dev/sda2, and /dev/sda3 partitions from Example 5.2, “Backing up Multiple ext4 Partitions”:
  1. Rebuild partitions you want to restore by using the fdisk command.
  2. Format the destination partitions:
    # mkfs.ext4 /dev/sda1
    # mkfs.ext4 /dev/sda2
    # mkfs.ext4 /dev/sda3
  3. Re-label all the partitions so they match the /etc/fstab file:
    # e2label /dev/sda1 Boot1
    # e2label /dev/sda2 Root
    # e2label /dev/sda3 Data
  4. Prepare the working directories.
    Mount the new partitions:
    # mkdir /mnt/sda1
    # mount -t ext4 /dev/sda1 /mnt/sda1
    # mkdir /mnt/sda2
    # mount -t ext4 /dev/sda2 /mnt/sda2
    # mkdir /mnt/sda3
    # mount -t ext4 /dev/sda3 /mnt/sda3
    Mount the partition that contains backup files:
    # mkdir /backup-files
    # mount -t ext4 /dev/sda6 /backup-files
  5. Restore the data from backup to the mounted partitions:
    # cd /mnt/sda1
    # restore -rf /backup-files/sda1.dump
    # cd /mnt/sda2
    # restore -rf /backup-files/sda2.dump
    # cd /mnt/sda3
    # restore -rf /backup-files/sda3.dump
  6. Reboot:
    # systemctl reboot

Additional Resources

  • For more information, see the restore(8) man page.

5.6. Other ext4 File System Utilities

Red Hat Enterprise Linux 7 also features other utilities for managing ext4 file systems:
e2fsck
Used to repair an ext4 file system. This tool checks and repairs an ext4 file system more efficiently than ext3, thanks to updates in the ext4 disk structure.
e2label
Changes the label on an ext4 file system. This tool also works on ext2 and ext3 file systems.
quota
Controls and reports on disk space (blocks) and file (inode) usage by users and groups on an ext4 file system. For more information on using quota, refer to man quota and Section 17.1, “Configuring Disk Quotas”.
fsfreeze
To suspend access to a file system, use the command # fsfreeze -f mount-point to freeze it and # fsfreeze -u mount-point to unfreeze it. This halts access to the file system and creates a stable image on disk.

Note

It is unnecessary to use fsfreeze for device-mapper drives.
For more information see the fsfreeze(8) manpage.
As demonstrated in Section 5.2, “Mounting an ext4 File System”, the tune2fs utility can also adjust configurable file system parameters for ext2, ext3, and ext4 file systems. In addition, the following tools are also useful in debugging and analyzing ext4 file systems:
debugfs
Debugs ext2, ext3, or ext4 file systems.
e2image
Saves critical ext2, ext3, or ext4 file system metadata to a file.
For more information about these utilities, refer to their respective man pages.

Chapter 6. Btrfs (Technology Preview)

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.
Btrfs is a next generation Linux file system that offers advanced management, reliability, and scalability features. It is unique in offering snapshots, compression, and integrated device management.

6.1. Creating a btrfs File System

In order to make a basic btrfs file system, use the following command:
# mkfs.btrfs /dev/device
For more information on creating btrfs file systems with added devices and specifying multi-device profiles for metadata and data, refer to Section 6.4, “Integrated Volume Management of Multiple Devices”.

6.2. Mounting a btrfs file system

To mount any device in the btrfs file system use the following command:
# mount /dev/device /mount-point
Other useful mount options include:
device=/dev/name
Appending this option to the mount command tells btrfs to scan the named device for a btrfs volume. This is used to ensure the mount will succeed as attempting to mount devices that are not btrfs will cause the mount to fail.

Note

This does not mean all devices will be added to the file system, it only scans them.
max_inline=number
Use this option to set the maximum amount of space (in bytes) that can be used to inline data within a metadata B-tree leaf. The default is 8192 bytes. For 4k pages it is limited to 3900 bytes due to additional headers that need to fit into the leaf.
alloc_start=number
Use this option to set where in the disk allocations start.
thread_pool=number
Use this option to assign the number of worker threads allocated.
discard
Use this option to enable discard/TRIM on freed blocks.
noacl
Use this option to disable the use of ACL's.
space_cache
Use this option to store the free space data on disk to make caching a block group faster. This is a persistent change and is safe to boot into old kernels.
nospace_cache
Use this option to disable the above space_cache.
clear_cache
Use this option to clear all the free space caches during mount. This is a safe option but will trigger the space cache to be rebuilt. As such, leave the file system mounted in order to let the rebuild process finish. This mount option is intended to be used once and only after problems are apparent with the free space.
enospc_debug
This option is used to debug problems with "no space left".
recovery
Use this option to enable autorecovery upon mount.

6.3. Resizing a btrfs File System

It is not possible to resize a btrfs file system but it is possible to resize each of the devices it uses. If there is only one device in use then this works the same as resizing the file system. If there are multiple devices in use then they must be manually resized to achieve the desired result.

Note

The unit size is not case specific; it accepts both G or g for GiB.
The command does not accept t for terabytes or p for petabytes. It only accepts k, m, and g.

Enlarging a btrfs File System

To enlarge the file system on a single device, use the command:
# btrfs filesystem resize amount /mount-point
For example:
# btrfs filesystem resize +200M /btrfssingle
Resize '/btrfssingle' of '+200M'
To enlarge a multi-device file system, the device to be enlarged must be specified. First, show all devices that have a btrfs file system at a specified mount point:
# btrfs filesystem show /mount-point
For example:
# btrfs filesystem show /btrfstest
Label: none  uuid: 755b41b7-7a20-4a24-abb3-45fdbed1ab39
	Total devices 4 FS bytes used 192.00KiB
	devid    1 size 1.00GiB used 224.75MiB path /dev/vdc
	devid    2 size 524.00MiB used 204.75MiB path /dev/vdd
	devid    3 size 1.00GiB used 8.00MiB path /dev/vde
	devid    4 size 1.00GiB used 8.00MiB path /dev/vdf

Btrfs v3.16.2
Then, after identifying the devid of the device to be enlarged, use the following command:
# btrfs filesystem resize devid:amount /mount-point
For example:
# btrfs filesystem resize 2:+200M /btrfstest
Resize '/btrfstest/' of '2:+200M'

Note

The amount can also be max instead of a specified amount. This will use all remaining free space on the device.

Shrinking a btrfs File System

To shrink the file system on a single device, use the command:
# btrfs filesystem resize amount /mount-point
For example:
# btrfs filesystem resize -200M /btrfssingle
Resize '/btrfssingle' of '-200M'
To shrink a multi-device file system, the device to be shrunk must be specified. First, show all devices that have a btrfs file system at a specified mount point:
# btrfs filesystem show /mount-point
For example:
# btrfs filesystem show /btrfstest
Label: none  uuid: 755b41b7-7a20-4a24-abb3-45fdbed1ab39
	Total devices 4 FS bytes used 192.00KiB
	devid    1 size 1.00GiB used 224.75MiB path /dev/vdc
	devid    2 size 524.00MiB used 204.75MiB path /dev/vdd
	devid    3 size 1.00GiB used 8.00MiB path /dev/vde
	devid    4 size 1.00GiB used 8.00MiB path /dev/vdf

Btrfs v3.16.2
Then, after identifying the devid of the device to be shrunk, use the following command:
# btrfs filesystem resize devid:amount /mount-point
For example:
# btrfs filesystem resize 2:-200M /btrfstest
Resize '/btrfstest' of '2:-200M'

Set the File System Size

To set the file system to a specific size on a single device, use the command:
# btrfs filesystem resize amount /mount-point
For example:
# btrfs filesystem resize 700M /btrfssingle
Resize '/btrfssingle' of '700M'
To set the file system size of a multi-device file system, the device to be changed must be specified. First, show all devices that have a btrfs file system at the specified mount point:
# btrfs filesystem show /mount-point
For example:
# btrfs filesystem show /btrfstest
Label: none  uuid: 755b41b7-7a20-4a24-abb3-45fdbed1ab39
	Total devices 4 FS bytes used 192.00KiB
	devid    1 size 1.00GiB used 224.75MiB path /dev/vdc
	devid    2 size 724.00MiB used 204.75MiB path /dev/vdd
	devid    3 size 1.00GiB used 8.00MiB path /dev/vde
	devid    4 size 1.00GiB used 8.00MiB path /dev/vdf

Btrfs v3.16.2
Then, after identifying the devid of the device to be changed, use the following command:
# btrfs filesystem resize devid:amount /mount-point
For example:
# btrfs filesystem resize 2:300M /btrfstest
Resize '/btrfstest' of '2:300M'

6.4. Integrated Volume Management of Multiple Devices

A btrfs file system can be created on top of many devices, and more devices can be added after the file system has been created. By default, metadata will be mirrored across two devices and data will be striped across all devices present, however if only one device is present, metadata will be duplicated on that device.

6.4.1. Creating a File System with Multiple Devices

The mkfs.btrfs command, as detailed in Section 6.1, “Creating a btrfs File System”, accepts the options -d for data, and -m for metadata. Valid specifications are:
  • raid0
  • raid1
  • raid10
  • dup
  • single
The -m single option instructs that no duplication of metadata is done. This may be desired when using hardware raid.

Note

RAID 10 requires at least four devices to run correctly.

Example 6.1. Creating a RAID 10 btrfs File System

Create a file system across four devices (metadata mirrored, data striped).
# mkfs.btrfs /dev/device1 /dev/device2 /dev/device3 /dev/device4
Stripe the metadata without mirroring.
# mkfs.btrfs -m raid0 /dev/device1 /dev/device2
Use raid10 for both data and metadata.
# mkfs.btrfs -m raid10 -d raid10 /dev/device1 /dev/device2 /dev/device3 /dev/device4
Do not duplicate metadata on a single drive.
# mkfs.btrfs -m single /dev/device
Use the single option to use the full capacity of each drive when the drives are different sizes.
# mkfs.btrfs -d single /dev/device1 /dev/device2 /dev/device3
To add a new device to an already created multi-device file system, use the following command:
# btrfs device add /dev/device1 /mount-point
After rebooting or reloading the btrfs module, use the btrfs device scan command to discover all multi-device file systems. See Section 6.4.2, “Scanning for btrfs Devices” for more information.

6.4.2. Scanning for btrfs Devices

Use btrfs device scan to scan all block devices under /dev and probe for btrfs volumes. This must be performed after loading the btrfs module if running with more than one device in a file system.
To scan all devices, use the following command:
# btrfs device scan
To scan a single device, use the following command:
# btrfs device scan /dev/device

6.4.3. Adding New Devices to a btrfs File System

Use the btrfs filesystem show command to list all the btrfs file systems and which devices they include.
The btrfs device add command is used to add new devices to a mounted file system.
The btrfs filesystem balance command balances (restripes) the allocated extents across all existing devices.
An example of all these commands together to add a new device is as follows:

Example 6.2. Add a New Device to a btrfs File System

First, create and mount a btrfs file system. Refer to Section 6.1, “Creating a btrfs File System” for more information on how to create a btrfs file system, and to Section 6.2, “Mounting a btrfs file system” for more information on how to mount a btrfs file system.
# mkfs.btrfs /dev/device1
# mount /dev/device1
Next, add a second device to the mounted btrfs file system.
# btrfs device add /dev/device2 /mount-point
The metadata and data on these devices are still stored only on /dev/device1. It must now be balanced to spread across all devices.
# btrfs filesystem balance /mount-point
Balancing a file system will take some time as it reads all of the file system's data and metadata and rewrites it across the new device.

6.4.4. Converting a btrfs File System

To convert a non-raid file system to a raid, add a device and run a balance filter that changes the chunk allocation profile.

Example 6.3. Converting a btrfs File System

To convert an existing single device system, /dev/sdb1 in this case, into a two device, raid1 system in order to protect against a single disk failure, use the following commands:
# mount /dev/sdb1 /mnt
# btrfs device add /dev/sdc1 /mnt
# btrfs balance start -dconvert=raid1 -mconvert=raid1 /mnt

Important

If the metadata is not converted from the single-device default, it remains as DUP. This does not guarantee that copies of the block are on separate devices. If data is not converted it does not have any redundant copies at all.

6.4.5. Removing btrfs Devices

Use the btrfs device delete command to remove an online device. It redistributes any extents in use to other devices in the file system in order to be safely removed.

Example 6.4. Removing a Device on a btrfs File System

First create and mount a few btrfs file systems.
# mkfs.btrfs /dev/sdb /dev/sdc /dev/sdd /dev/sde
# mount /dev/sdb /mnt
Add some data to the file system.
Finally, remove the required device.
# btrfs device delete /dev/sdc /mnt

6.4.6. Replacing Failed Devices on a btrfs File System

Section 6.4.5, “Removing btrfs Devices” can be used to remove a failed device provided the super block can still be read. However, if a device is missing or the super block corrupted, the file system will need to be mounted in a degraded mode:
# mkfs.btrfs -m raid1 /dev/sdb /dev/sdc /dev/sdd /dev/sde


  ssd is destroyed or removed, use -o degraded to force the mount
  to ignore missing devices


# mount -o degraded /dev/sdb /mnt


  'missing' is a special device name


# btrfs device delete missing /mnt
The command btrfs device delete missing removes the first device that is described by the file system metadata but not present when the file system was mounted.

Important

It is impossible to go below the minimum number of devices required for the specific raid layout, even including the missing one. It may be required to add a new device in order to remove the failed one.
For example, for a raid1 layout with two devices, if a device fails it is required to:
  1. mount in degraded mode,
  2. add a new device,
  3. and, remove the missing device.

6.4.7. Registering a btrfs File System in /etc/fstab

If you do not have an initrd or it does not perform a btrfs device scan, it is possible to mount a multi-volume btrfs file system by passing all the devices in the file system explicitly to the mount command.

Example 6.5. Example /etc/fstab Entry

An example of a suitable /etc/fstab entry would be:
/dev/sdb    /mnt    btrfs    device=/dev/sdb,device=/dev/sdc,device=/dev/sdd,device=/dev/sde    0
Note that using universally unique identifiers (UUIDs) also works and is more stable than using device paths.

6.5. SSD Optimization

Using the btrfs file system can optimize SSD. There are two ways this can be done.
The first way is mkfs.btrfs turns off metadata duplication on a single device when /sys/block/device/queue/rotational is zero for the single specified device. This is equivalent to specifying -m single on the command line. It can be overridden and duplicate metadata forced by providing the -m dup option. Duplication is not required due to SSD firmware potentially losing both copies. This wastes space and is a performance cost.
The second way is through a group of SSD mount options: ssd, nossd, and ssd_spread.
The ssd option does several things:
  • It allows larger metadata cluster allocation.
  • It allocates data more sequentially where possible.
  • It disables btree leaf rewriting to match key and block order.
  • It commits log fragments without batching multiple processes.

Note

The ssd mount option only enables the ssd option. Use the nossd option to disable it.
Some SSDs perform best when reusing block numbers often, while others perform much better when clustering strictly allocates big chunks of unused space. By default, mount -o ssd will find groupings of blocks where there are several free blocks that might have allocated blocks mixed in. The command mount -o ssd_spread ensures there are no allocated blocks mixed in. This improves performance on lower end SSDs.

Note

The ssd_spread option enables both the ssd and the ssd_spread options. Use the nossd to disable both these options.
The ssd_spread option is never automatically set if none of the ssd options are provided and any of the devices are non-rotational.
These options will all need to be tested with your specific build to see if their use improves or reduces performance, as each combination of SSD firmware and application loads are different.

6.6. btrfs References

The man page btrfs(8) covers all important management commands. In particular this includes:
  • All the subvolume commands for managing snapshots.
  • The device commands for managing devices.
  • The scrub, balance, and defragment commands.
The man page mkfs.btrfs(8) contains information on creating a btrfs file system including all the options regarding it.
The man page btrfsck(8) for information regarding fsck on btrfs systems.

Chapter 7. Global File System 2

The Red Hat Global File System 2 (GFS2) is a native file system that interfaces directly with the Linux kernel file system interface (VFS layer). When implemented as a cluster file system, GFS2 employs distributed metadata and multiple journals.
GFS2 is based on 64-bit architecture, which can theoretically accommodate an 8 exabyte file system. However, the current supported maximum size of a GFS2 file system is 100 TB. If a system requires GFS2 file systems larger than 100 TB, contact your Red Hat service representative.
When determining the size of a file system, consider its recovery needs. Running the fsck command on a very large file system can take a long time and consume a large amount of memory. Additionally, in the event of a disk or disk-subsystem failure, recovery time is limited by the speed of backup media.
When configured in a Red Hat Cluster Suite, Red Hat GFS2 nodes can be configured and managed with Red Hat Cluster Suite configuration and management tools. Red Hat GFS2 then provides data sharing among GFS2 nodes in a Red Hat cluster, with a single, consistent view of the file system namespace across the GFS2 nodes. This allows processes on different nodes to share GFS2 files in the same way that processes on the same node can share files on a local file system, with no discernible difference. For information about the Red Hat Cluster Suite, see Red Hat's Cluster Administration guide.
A GFS2 must be built on a logical volume (created with LVM) that is a linear or mirrored volume. Logical volumes created with LVM in a Red Hat Cluster suite are managed with CLVM (a cluster-wide implementation of LVM), enabled by the CLVM daemon clvmd, and running in a Red Hat Cluster Suite cluster. The daemon makes it possible to use LVM2 to manage logical volumes across a cluster, allowing all nodes in the cluster to share the logical volumes. For information on the Logical Volume Manager, see Red Hat's Logical Volume Manager Administration guide.
The gfs2.ko kernel module implements the GFS2 file system and is loaded on GFS2 cluster nodes.
For comprehensive information on the creation and configuration of GFS2 file systems in clustered and non-clustered storage, see Red Hat's Global File System 2 guide.

Chapter 8. Network File System (NFS)

A Network File System (NFS) allows remote hosts to mount file systems over a network and interact with those file systems as though they are mounted locally. This enables system administrators to consolidate resources onto centralized servers on the network.
This chapter focuses on fundamental NFS concepts and supplemental information.

8.1. Introduction to NFS

Currently, there are two major versions of NFS included in Red Hat Enterprise Linux:
  • NFS version 3 (NFSv3) supports safe asynchronous writes and is more robust at error handling than the previous NFSv2. It also supports 64-bit file sizes and offsets, allowing clients to access more than 2 GB of file data.
  • NFS version 4 (NFSv4) works through firewalls and on the Internet, no longer requires an rpcbind service, supports ACLs, and utilizes stateful operations.
Red Hat Enterprise Linux fully supports NFS version 4.2 (NFSv4.2) since the Red Hat Enterprise Linux 7.4 release.
Following are the features of NFSv4.2 in Red Hat Enterprise Linux :
  • Sparse Files: It verifies space efficiency of a file and allows placeholder to improve storage efficiency. It is a file having one or more holes; holes are unallocated or uninitialized data blocks consisting only of zeroes. lseek() operation in NFSv4.2, supports seek_hole() and seek_data(), which allows application to map out the location of holes in the sparse file.
  • Space Reservation: It permits storage servers to reserve free space, which prohibits servers to run out of space. NFSv4.2 supports allocate() operation to reserve space, deallocate() operation to unreserve space, and fallocate() operation to preallocate or deallocate space in a file.
  • Labeled NFS: It enforces data access rights and enables SELinux labels between a client and a server for individual files on an NFS file system.
  • Layout Enhancements: NFSv4.2 provides new operation, layoutstats(), which the client can use to notify the metadata server about its communication with the layout.
Versions of Red Hat Enterprise Linux earlier than 7.4 support NFS up to version 4.1.
Following are the features of NFSv4.1:
  • Enhances performance and security of network, and also includes client-side support for Parallel NFS (pNFS).
  • No longer requires a separate TCP connection for callbacks, which allows an NFS server to grant delegations even when it cannot contact the client. For example, when NAT or a firewall interferes.
  • It provides exactly once semantics (except for reboot operations), preventing a previous issue whereby certain operations could return an inaccurate result if a reply was lost and the operation was sent twice.
NFS clients attempt to mount using NFSv4.1 by default, and fall back to NFSv4.0 when the server does not support NFSv4.1. The mount later fall back to NFSv3 when server does not support NFSv4.0.

Note

NFS version 2 (NFSv2) is no longer supported by Red Hat.
All versions of NFS can use Transmission Control Protocol (TCP) running over an IP network, with NFSv4 requiring it. NFSv3 can use the User Datagram Protocol (UDP) running over an IP network to provide a stateless network connection between the client and server.
When using NFSv3 with UDP, the stateless UDP connection (under normal conditions) has less protocol overhead than TCP. This can translate into better performance on very clean, non-congested networks. However, because UDP is stateless, if the server goes down unexpectedly, UDP clients continue to saturate the network with requests for the server. In addition, when a frame is lost with UDP, the entire RPC request must be retransmitted; with TCP, only the lost frame needs to be resent. For these reasons, TCP is the preferred protocol when connecting to an NFS server.
The mounting and locking protocols have been incorporated into the NFSv4 protocol. The server also listens on the well-known TCP port 2049. As such, NFSv4 does not need to interact with rpcbind [1], lockd, and rpc.statd daemons. The rpc.mountd daemon is still required on the NFS server to set up the exports, but is not involved in any over-the-wire operations.

Note

TCP is the default transport protocol for NFS version 3 under Red Hat Enterprise Linux. UDP can be used for compatibility purposes as needed, but is not recommended for wide usage. NFSv4 requires TCP.
All the RPC/NFS daemons have a '-p' command line option that can set the port, making firewall configuration easier.
After TCP wrappers grant access to the client, the NFS server refers to the /etc/exports configuration file to determine whether the client is allowed to access any exported file systems. Once verified, all file and directory operations are available to the user.

Important

In order for NFS to work with a default installation of Red Hat Enterprise Linux with a firewall enabled, configure IPTables with the default TCP port 2049. Without proper IPTables configuration, NFS will not function properly.
The NFS initialization script and rpc.nfsd process now allow binding to any specified port during system start up. However, this can be error-prone if the port is unavailable, or if it conflicts with another daemon.

8.1.1. Required Services

Red Hat Enterprise Linux uses a combination of kernel-level support and daemon processes to provide NFS file sharing. All NFS versions rely on Remote Procedure Calls (RPC) between clients and servers. RPC services under Red Hat Enterprise Linux 7 are controlled by the rpcbind service. To share or mount NFS file systems, the following services work together depending on which version of NFS is implemented:

Note

The portmap service was used to map RPC program numbers to IP address port number combinations in earlier versions of Red Hat Enterprise Linux. This service is now replaced by rpcbind in Red Hat Enterprise Linux 7 to enable IPv6 support.
nfs
systemctl start nfs starts the NFS server and the appropriate RPC processes to service requests for shared NFS file systems.
nfslock
systemctl start nfs-lock activates a mandatory service that starts the appropriate RPC processes allowing NFS clients to lock files on the server.
rpcbind
rpcbind accepts port reservations from local RPC services. These ports are then made available (or advertised) so the corresponding remote RPC services can access them. rpcbind responds to requests for RPC services and sets up connections to the requested RPC service. This is not used with NFSv4.
The following RPC processes facilitate NFS services:
rpc.mountd
This process is used by an NFS server to process MOUNT requests from NFSv3 clients. It checks that the requested NFS share is currently exported by the NFS server, and that the client is allowed to access it. If the mount request is allowed, the rpc.mountd server replies with a Success status and provides the File-Handle for this NFS share back to the NFS client.
rpc.nfsd
rpc.nfsd allows explicit NFS versions and protocols the server advertises to be defined. It works with the Linux kernel to meet the dynamic demands of NFS clients, such as providing server threads each time an NFS client connects. This process corresponds to the nfs service.
lockd
lockd is a kernel thread which runs on both clients and servers. It implements the Network Lock Manager (NLM) protocol, which allows NFSv3 clients to lock files on the server. It is started automatically whenever the NFS server is run and whenever an NFS file system is mounted.
rpc.statd
This process implements the Network Status Monitor (NSM) RPC protocol, which notifies NFS clients when an NFS server is restarted without being gracefully brought down. rpc.statd is started automatically by the nfslock service, and does not require user configuration. This is not used with NFSv4.
rpc.rquotad
This process provides user quota information for remote users. rpc.rquotad is started automatically by the nfs service and does not require user configuration.
rpc.idmapd
rpc.idmapd provides NFSv4 client and server upcalls, which map between on-the-wire NFSv4 names (strings in the form of user@domain) and local UIDs and GIDs. For idmapd to function with NFSv4, the /etc/idmapd.conf file must be configured. At a minimum, the "Domain" parameter should be specified, which defines the NFSv4 mapping domain. If the NFSv4 mapping domain is the same as the DNS domain name, this parameter can be skipped. The client and server must agree on the NFSv4 mapping domain for ID mapping to function properly.

Note

In Red Hat Enterprise Linux 7, only the NFSv4 server uses rpc.idmapd. The NFSv4 client uses the keyring-based idmapper nfsidmap. nfsidmap is a stand-alone program that is called by the kernel on-demand to perform ID mapping; it is not a daemon. If there is a problem with nfsidmap does the client fall back to using rpc.idmapd. More information regarding nfsidmap can be found on the nfsidmap man page.

8.2. Configuring NFS Client

The mount command mounts NFS shares on the client side. Its format is as follows:
# mount -t nfs -o options server:/remote/export /local/directory
This command uses the following variables:
options
A comma-delimited list of mount options; for more information on valid NFS mount options, see Section 8.4, “Common NFS Mount Options”.
server
The hostname, IP address, or fully qualified domain name of the server exporting the file system you wish to mount
/remote/export
The file system or directory being exported from the server, that is, the directory you wish to mount
/local/directory
The client location where /remote/export is mounted
The NFS protocol version used in Red Hat Enterprise Linux 7 is identified by the mount options nfsvers or vers. By default, mount uses NFSv4 with mount -t nfs. If the server does not support NFSv4, the client automatically steps down to a version supported by the server. If the nfsvers/vers option is used to pass a particular version not supported by the server, the mount fails. The file system type nfs4 is also available for legacy reasons; this is equivalent to running mount -t nfs -o nfsvers=4 host:/remote/export /local/directory.
For more information, see man mount.
If an NFS share was mounted manually, the share will not be automatically mounted upon reboot. Red Hat Enterprise Linux offers two methods for mounting remote file systems automatically at boot time: the /etc/fstab file and the autofs service. For more information, see Section 8.2.1, “Mounting NFS File Systems Using /etc/fstab and Section 8.3, “autofs.

8.2.1. Mounting NFS File Systems Using /etc/fstab

An alternate way to mount an NFS share from another machine is to add a line to the /etc/fstab file. The line must state the hostname of the NFS server, the directory on the server being exported, and the directory on the local machine where the NFS share is to be mounted. You must be root to modify the /etc/fstab file.

Example 8.1. Syntax Example

The general syntax for the line in /etc/fstab is as follows:
server:/usr/local/pub    /pub   nfs    defaults 0 0
The mount point /pub must exist on the client machine before this command can be executed. After adding this line to /etc/fstab on the client system, use the command mount /pub, and the mount point /pub is mounted from the server.
A valid /etc/fstab entry to mount an NFS export should contain the following information:
server:/remote/export /local/directory nfs options 0 0
The variables server, /remote/export, /local/directory, and options are the same ones used when manually mounting an NFS share. For more information, see Section 8.2, “Configuring NFS Client”.

Note

The mount point /local/directory must exist on the client before /etc/fstab is read. Otherwise, the mount fails.
After editing /etc/fstab, regenerate mount units so that your system registers the new configuration:
# systemctl daemon-reload
Additional Resources
  • For more information about /etc/fstab, refer to man fstab.

8.3. autofs

One drawback of using /etc/fstab is that, regardless of how infrequently a user accesses the NFS mounted file system, the system must dedicate resources to keep the mounted file system in place. This is not a problem with one or two mounts, but when the system is maintaining mounts to many systems at one time, overall system performance can be affected. An alternative to /etc/fstab is to use the kernel-based automount utility. An automounter consists of two components:
  • a kernel module that implements a file system, and
  • a user-space daemon that performs all of the other functions.
The automount utility can mount and unmount NFS file systems automatically (on-demand mounting), therefore saving system resources. It can be used to mount other file systems including AFS, SMBFS, CIFS, and local file systems.

Important

The nfs-utils package is now a part of both the 'NFS file server' and the 'Network File System Client' groups. As such, it is no longer installed by default with the Base group. Ensure that nfs-utils is installed on the system first before attempting to automount an NFS share.
autofs is also part of the 'Network File System Client' group.
autofs uses /etc/auto.master (master map) as its default primary configuration file. This can be changed to use another supported network source and name using the autofs configuration (in /etc/sysconfig/autofs) in conjunction with the Name Service Switch (NSS) mechanism. An instance of the autofs version 4 daemon was run for each mount point configured in the master map and so it could be run manually from the command line for any given mount point. This is not possible with autofs version 5, because it uses a single daemon to manage all configured mount points; as such, all automounts must be configured in the master map. This is in line with the usual requirements of other industry standard automounters. Mount point, hostname, exported directory, and options can all be specified in a set of files (or other supported network sources) rather than configuring them manually for each host.

8.3.1. Improvements in autofs Version 5 over Version 4

autofs version 5 features the following enhancements over version 4:
Direct map support
Direct maps in autofs provide a mechanism to automatically mount file systems at arbitrary points in the file system hierarchy. A direct map is denoted by a mount point of /- in the master map. Entries in a direct map contain an absolute path name as a key (instead of the relative path names used in indirect maps).
Lazy mount and unmount support
Multi-mount map entries describe a hierarchy of mount points under a single key. A good example of this is the -hosts map, commonly used for automounting all exports from a host under /net/host as a multi-mount map entry. When using the -hosts map, an ls of /net/host will mount autofs trigger mounts for each export from host. These will then mount and expire them as they are accessed. This can greatly reduce the number of active mounts needed when accessing a server with a large number of exports.
Enhanced LDAP support
The autofs configuration file (/etc/sysconfig/autofs) provides a mechanism to specify the autofs schema that a site implements, thus precluding the need to determine this via trial and error in the application itself. In addition, authenticated binds to the LDAP server are now supported, using most mechanisms supported by the common LDAP server implementations. A new configuration file has been added for this support: /etc/autofs_ldap_auth.conf. The default configuration file is self-documenting, and uses an XML format.
Proper use of the Name Service Switch (nsswitch) configuration.
The Name Service Switch configuration file exists to provide a means of determining from where specific configuration data comes. The reason for this configuration is to allow administrators the flexibility of using the back-end database of choice, while maintaining a uniform software interface to access the data. While the version 4 automounter is becoming increasingly better at handling the NSS configuration, it is still not complete. Autofs version 5, on the other hand, is a complete implementation.
For more information on the supported syntax of this file, see man nsswitch.conf. Not all NSS databases are valid map sources and the parser will reject ones that are invalid. Valid sources are files, yp, nis, nisplus, ldap, and hesiod.
Multiple master map entries per autofs mount point
One thing that is frequently used but not yet mentioned is the handling of multiple master map entries for the direct mount point /-. The map keys for each entry are merged and behave as one map.

Example 8.2. Multiple Master Map Entries per autofs Mount Point

Following is an example in the connectathon test maps for the direct mounts:
/- /tmp/auto_dcthon
/- /tmp/auto_test3_direct
/- /tmp/auto_test4_direct
 

8.3.2. Configuring autofs

The primary configuration file for the automounter is /etc/auto.master, also referred to as the master map which may be changed as described in the Section 8.3.1, “Improvements in autofs Version 5 over Version 4”. The master map lists autofs-controlled mount points on the system, and their corresponding configuration files or network sources known as automount maps. The format of the master map is as follows:
mount-point map-name options
The variables used in this format are:
mount-point
The autofs mount point, /home, for example.
map-name
The name of a map source which contains a list of mount points, and the file system location from which those mount points should be mounted.
options
If supplied, these applies to all entries in the given map provided they do not themselves have options specified. This behavior is different from autofs version 4 where options were cumulative. This has been changed to implement mixed environment compatibility.

Example 8.3. /etc/auto.master File

The following is a sample line from /etc/auto.master file (displayed with cat /etc/auto.master):
/home /etc/auto.misc
The general format of maps is similar to the master map, however the "options" appear between the mount point and the location instead of at the end of the entry as in the master map:
mount-point   [options]   location
The variables used in this format are:
mount-point
This refers to the autofs mount point. This can be a single directory name for an indirect mount or the full path of the mount point for direct mounts. Each direct and indirect map entry key (mount-point) may be followed by a space separated list of offset directories (subdirectory names each beginning with a /) making them what is known as a multi-mount entry.
options
Whenever supplied, these are the mount options for the map entries that do not specify their own options.
location
This refers to the file system location such as a local file system path (preceded with the Sun map format escape character ":" for map names beginning with /), an NFS file system or other valid file system location.
The following is a sample of contents from a map file (for example, /etc/auto.misc):
payroll -fstype=nfs personnel:/dev/hda3
sales -fstype=ext3 :/dev/hda4
The first column in a map file indicates the autofs mount point (sales and payroll from the server called personnel). The second column indicates the options for the autofs mount while the third column indicates the source of the mount. Following the given configuration, the autofs mount points will be /home/payroll and /home/sales. The -fstype= option is often omitted and is generally not needed for correct operation.
The automounter create the directories if they do not exist. If the directories exist before the automounter was started, the automounter will not remove them when it exits.
To start the automount daemon, use the following command:
# systemctl start autofs
To restart the automount daemon, use the following command:
# systemctl restart autofs
Using the given configuration, if a process requires access to an autofs unmounted directory such as /home/payroll/2006/July.sxc, the automount daemon automatically mounts the directory. If a timeout is specified, the directory is automatically unmounted if the directory is not accessed for the timeout period.
To view the status of the automount daemon, use the following command:
# systemctl status autofs

8.3.3. Overriding or Augmenting Site Configuration Files

It can be useful to override site defaults for a specific mount point on a client system. For example, consider the following conditions:
  • Automounter maps are stored in NIS and the /etc/nsswitch.conf file has the following directive:
    automount:    files nis
  • The auto.master file contains:
    +auto.master
  • The NIS auto.master map file contains:
    /home auto.home
  • The NIS auto.home map contains:
    beth        fileserver.example.com:/export/home/beth
    joe        fileserver.example.com:/export/home/joe
    *       fileserver.example.com:/export/home/&
  • The file map /etc/auto.home does not exist.
Given these conditions, let's assume that the client system needs to override the NIS map auto.home and mount home directories from a different server. In this case, the client needs to use the following /etc/auto.master map:
/home ­/etc/auto.home
+auto.master
The /etc/auto.home map contains the entry:
*    labserver.example.com:/export/home/&
Because the automounter only processes the first occurrence of a mount point, /home contain the contents of /etc/auto.home instead of the NIS auto.home map.
Alternatively, to augment the site-wide auto.home map with just a few entries, create an /etc/auto.home file map, and in it put the new entries. At the end, include the NIS auto.home map. Then the /etc/auto.home file map looks similar to:
mydir someserver:/export/mydir
+auto.home
With these NIS auto.home map conditions, the ls /home command outputs:
beth joe mydir
This last example works as expected because autofs does not include the contents of a file map of the same name as the one it is reading. As such, autofs moves on to the next map source in the nsswitch configuration.

8.3.4. Using LDAP to Store Automounter Maps

LDAP client libraries must be installed on all systems configured to retrieve automounter maps from LDAP. On Red Hat Enterprise Linux, the openldap package should be installed automatically as a dependency of the automounter. To configure LDAP access, modify /etc/openldap/ldap.conf. Ensure that BASE, URI, and schema are set appropriately for your site.
The most recently established schema for storing automount maps in LDAP is described by rfc2307bis. To use this schema it is necessary to set it in the autofs configuration (/etc/sysconfig/autofs) by removing the comment characters from the schema definition. For example:

Example 8.4. Setting autofs Configuration

DEFAULT_MAP_OBJECT_CLASS="automountMap"
DEFAULT_ENTRY_OBJECT_CLASS="automount"
DEFAULT_MAP_ATTRIBUTE="automountMapName"
DEFAULT_ENTRY_ATTRIBUTE="automountKey"
DEFAULT_VALUE_ATTRIBUTE="automountInformation"
Ensure that these are the only schema entries not commented in the configuration. The automountKey replaces the cn attribute in the rfc2307bis schema. Following is an example of an LDAP Data Interchange Format (LDIF) configuration:

Example 8.5. LDF Configuration

# extended LDIF
#
# LDAPv3
# base <> with scope subtree
# filter: (&(objectclass=automountMap)(automountMapName=auto.master))
# requesting: ALL
#

# auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: top
objectClass: automountMap
automountMapName: auto.master

# extended LDIF
#
# LDAPv3
# base <automountMapName=auto.master,dc=example,dc=com> with scope subtree
# filter: (objectclass=automount)
# requesting: ALL
#

# /home, auto.master, example.com
dn: automountMapName=auto.master,dc=example,dc=com
objectClass: automount
cn: /home

automountKey: /home
automountInformation: auto.home

# extended LDIF
#
# LDAPv3
# base <> with scope subtree
# filter: (&(objectclass=automountMap)(automountMapName=auto.home))
# requesting: ALL
#

# auto.home, example.com
dn: automountMapName=auto.home,dc=example,dc=com
objectClass: automountMap
automountMapName: auto.home

# extended LDIF
#
# LDAPv3
# base <automountMapName=auto.home,dc=example,dc=com> with scope subtree
# filter: (objectclass=automount)
# requesting: ALL
#

# foo, auto.home, example.com
dn: automountKey=foo,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: foo
automountInformation: filer.example.com:/export/foo

# /, auto.home, example.com
dn: automountKey=/,automountMapName=auto.home,dc=example,dc=com
objectClass: automount
automountKey: /
automountInformation: filer.example.com:/export/&

8.4. Common NFS Mount Options

Beyond mounting a file system with NFS on a remote host, it is also possible to specify other options at mount time to make the mounted share easier to use. These options can be used with manual mount commands, /etc/fstab settings, and autofs.
The following are options commonly used for NFS mounts:
lookupcache=mode
Specifies how the kernel should manage its cache of directory entries for a given mount point. Valid arguments for mode are all, none, or pos/positive.
nfsvers=version
Specifies which version of the NFS protocol to use, where version is 3 or 4. This is useful for hosts that run multiple NFS servers. If no version is specified, NFS uses the highest version supported by the kernel and mount command.
The option vers is identical to nfsvers, and is included in this release for compatibility reasons.
noacl
Turns off all ACL processing. This may be needed when interfacing with older versions of Red Hat Enterprise Linux, Red Hat Linux, or Solaris, since the most recent ACL technology is not compatible with older systems.
nolock
Disables file locking. This setting is sometimes required when connecting to very old NFS servers.
noexec
Prevents execution of binaries on mounted file systems. This is useful if the system is mounting a non-Linux file system containing incompatible binaries.
nosuid
Disables set-user-identifier or set-group-identifier bits. This prevents remote users from gaining higher privileges by running a setuid program.
port=num
Specifies the numeric value of the NFS server port. If num is 0 (the default value), then mount queries the remote host's rpcbind service for the port number to use. If the remote host's NFS daemon is not registered with its rpcbind service, the standard NFS port number of TCP 2049 is used instead.
rsize=num and wsize=num
These options set the maximum number of bytes to be transfered in a single NFS read or write operation.
There is no fixed default value for rsize and wsize. By default, NFS uses the largest possible value that both the server and the client support. In Red Hat Enterprise Linux 7, the client and server maximum is 1,048,576 bytes. For more details, see the What are the default and maximum values for rsize and wsize with NFS mounts? KBase article.
sec=flavors
Security flavors to use for accessing files on the mounted export. The flavors value is a colon-separated list of one or more security flavors.
By default, the client attempts to find a security flavor that both the client and the server support. If the server does not support any of the selected flavors, the mount operation fails.
sec=sys uses local UNIX UIDs and GIDs. These use AUTH_SYS to authenticate NFS operations.
sec=krb5 uses Kerberos V5 instead of local UNIX UIDs and GIDs to authenticate users.
sec=krb5i uses Kerberos V5 for user authentication and performs integrity checking of NFS operations using secure checksums to prevent data tampering.
sec=krb5p uses Kerberos V5 for user authentication, integrity checking, and encrypts NFS traffic to prevent traffic sniffing. This is the most secure setting, but it also involves the most performance overhead.
tcp
Instructs the NFS mount to use the TCP protocol.
udp
Instructs the NFS mount to use the UDP protocol.
For more information, see man mount and man nfs.

8.5. Starting and Stopping the NFS Server

Prerequisites

  • For servers that support NFSv2 or NFSv3 connections, the rpcbind[1] service must be running. To verify that rpcbind is active, use the following command:
    $ systemctl status rpcbind
    To configure an NFSv4-only server, which does not require rpcbind, see Section 8.6.7, “Configuring an NFSv4-only Server”.
  • On Red Hat Enterprise Linux 7.0, if your NFS server exports NFSv3 and is enabled to start at boot, you need to manually start and enable the nfs-lock service:
    # systemctl start nfs-lock
    # systemctl enable nfs-lock
    On Red Hat Enterprise Linux 7.1 and later, nfs-lock starts automatically if needed, and an attempt to enable it manually fails.

Procedures

  • To start an NFS server, use the following command:
    # systemctl start nfs
  • To enable NFS to start at boot, use the following command:
    # systemctl enable nfs
  • To stop the server, use:
    # systemctl stop nfs
  • The restart option is a shorthand way of stopping and then starting NFS. This is the most efficient way to make configuration changes take effect after editing the configuration file for NFS. To restart the server type:
    # systemctl restart nfs
  • After you edit the /etc/sysconfig/nfs file, restart the nfs-config service by running the following command for the new values to take effect:
    # systemctl restart nfs-config
  • The try-restart command only starts nfs if it is currently running. This command is the equivalent of condrestart (conditional restart) in Red Hat init scripts and is useful because it does not start the daemon if NFS is not running.
    To conditionally restart the server, type:
    # systemctl try-restart nfs
  • To reload the NFS server configuration file without restarting the service type:
    # systemctl reload nfs

8.6. Configuring the NFS Server

There are two ways to configure exports on an NFS server:
  • Manually editing the NFS configuration file, that is, /etc/exports, and
  • Through the command line, that is, by using the command exportfs

8.6.1. The /etc/exports Configuration File

The /etc/exports file controls which file systems are exported to remote hosts and specifies options. It follows the following syntax rules:
  • Blank lines are ignored.
  • To add a comment, start a line with the hash mark (#).
  • You can wrap long lines with a backslash (\).
  • Each exported file system should be on its own individual line.
  • Any lists of authorized hosts placed after an exported file system must be separated by space characters.
  • Options for each of the hosts must be placed in parentheses directly after the host identifier, without any spaces separating the host and the first parenthesis.
Each entry for an exported file system has the following structure:
export host(options)
The aforementioned structure uses the following variables:
export
The directory being exported
host
The host or network to which the export is being shared
options
The options to be used for host
It is possible to specify multiple hosts, along with specific options for each host. To do so, list them on the same line as a space-delimited list, with each hostname followed by its respective options (in parentheses), as in:
export host1(options1) host2(options2) host3(options3)
For information on different methods for specifying hostnames, see Section 8.6.5, “Hostname Formats”.
In its simplest form, the /etc/exports file only specifies the exported directory and the hosts permitted to access it, as in the following example:

Example 8.6. The /etc/exports File

/exported/directory bob.example.com
Here, bob.example.com can mount /exported/directory/ from the NFS server. Because no options are specified in this example, NFS uses default settings.
The default settings are:
ro
The exported file system is read-only. Remote hosts cannot change the data shared on the file system. To allow hosts to make changes to the file system (that is, read and write), specify the rw option.
sync
The NFS server will not reply to requests before changes made by previous requests are written to disk. To enable asynchronous writes instead, specify the option async.
wdelay
The NFS server will delay writing to the disk if it suspects another write request is imminent. This can improve performance as it reduces the number of times the disk must be accessed by separate write commands, thereby reducing write overhead. To disable this, specify the no_wdelay. no_wdelay is only available if the default sync option is also specified.
root_squash
This prevents root users connected remotely (as opposed to locally) from having root privileges; instead, the NFS server assigns them the user ID nfsnobody. This effectively "squashes" the power of the remote root user to the lowest local user, preventing possible unauthorized writes on the remote server. To disable root squashing, specify no_root_squash.
To squash every remote user (including root), use all_squash. To specify the user and group IDs that the NFS server should assign to remote users from a particular host, use the anonuid and anongid options, respectively, as in:
export host(anonuid=uid,anongid=gid)
Here, uid and gid are user ID number and group ID number, respectively. The anonuid and anongid options allow you to create a special user and group account for remote NFS users to share.
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. To disable this feature, specify the no_acl option when exporting the file system.
Each default for every exported file system must be explicitly overridden. For example, if the rw option is not specified, then the exported file system is shared as read-only. The following is a sample line from /etc/exports which overrides two default options:
/another/exported/directory 192.168.0.3(rw,async)
In this example 192.168.0.3 can mount /another/exported/directory/ read and write and all writes to disk are asynchronous. For more information on exporting options, see man exportfs.
Other options are available where no default value is specified. These include the ability to disable sub-tree checking, allow access from insecure ports, and allow insecure file locks (necessary for certain early NFS client implementations). For more information on these less-used options, see man exports.

Important

The format of the /etc/exports file is very precise, particularly in regards to use of the space character. Remember to always separate exported file systems from hosts and hosts from one another with a space character. However, there should be no other space characters in the file except on comment lines.
For example, the following two lines do not mean the same thing:
/home bob.example.com(rw)
/home bob.example.com (rw)
The first line allows only users from bob.example.com read and write access to the /home directory. The second line allows users from bob.example.com to mount the directory as read-only (the default), while the rest of the world can mount it read/write.

8.6.2. The exportfs Command

Every file system being exported to remote users with NFS, as well as the access level for those file systems, are listed in the /etc/exports file. When the nfs service starts, the /usr/sbin/exportfs command launches and reads this file, passes control to rpc.mountd (if NFSv3) for the actual mounting process, then to rpc.nfsd where the file systems are then available to remote users.
When issued manually, the /usr/sbin/exportfs command allows the root user to selectively export or unexport directories without restarting the NFS service. When given the proper options, the /usr/sbin/exportfs command writes the exported file systems to /var/lib/nfs/xtab. Since rpc.mountd refers to the xtab file when deciding access privileges to a file system, changes to the list of exported file systems take effect immediately.
The following is a list of commonly-used options available for /usr/sbin/exportfs:
-r
Causes all directories listed in /etc/exports to be exported by constructing a new export list in /var/lib/nfs/etab. This option effectively refreshes the export list with any changes made to /etc/exports.
-a
Causes all directories to be exported or unexported, depending on what other options are passed to /usr/sbin/exportfs. If no other options are specified, /usr/sbin/exportfs exports all file systems specified in /etc/exports.
-o file-systems
Specifies directories to be exported that are not listed in /etc/exports. Replace file-systems with additional file systems to be exported. These file systems must be formatted in the same way they are specified in /etc/exports. This option is often used to test an exported file system before adding it permanently to the list of file systems to be exported. For more information on /etc/exports syntax, see Section 8.6.1, “The /etc/exports Configuration File”.
-i
Ignores /etc/exports; only options given from the command line are used to define exported file systems.
-u
Unexports all shared directories. The command /usr/sbin/exportfs -ua suspends NFS file sharing while keeping all NFS daemons up. To re-enable NFS sharing, use exportfs -r.
-v
Verbose operation, where the file systems being exported or unexported are displayed in greater detail when the exportfs command is executed.
If no options are passed to the exportfs command, it displays a list of currently exported file systems. For more information about the exportfs command, see man exportfs.
8.6.2.1. Using exportfs with NFSv4
In Red Hat Enterprise Linux 7, no extra steps are required to configure NFSv4 exports as any filesystems mentioned are automatically available to NFSv3 and NFSv4 clients using the same path. This was not the case in previous versions.
To prevent clients from using NFSv4, turn it off by setting RPCNFSDARGS= -N 4 in /etc/sysconfig/nfs.

8.6.3. Running NFS Behind a Firewall

NFS requires rpcbind, which dynamically assigns ports for RPC services and can cause issues for configuring firewall rules. To allow clients to access NFS shares behind a firewall, edit the /etc/sysconfig/nfs file to set which ports the RPC services run on. To allow clients to access RPC Quota through a firewall, see Section 8.6.4, “Accessing RPC Quota through a Firewall”.
The /etc/sysconfig/nfs file does not exist by default on all systems. If /etc/sysconfig/nfs does not exist, create it and specify the following:
RPCMOUNTDOPTS="-p port"
This adds "-p port" to the rpc.mount command line: rpc.mount -p port.
To specify the ports to be used by the nlockmgr service, set the port number for the nlm_tcpport and nlm_udpport options in the /etc/modprobe.d/lockd.conf file.
If NFS fails to start, check /var/log/messages. Commonly, NFS fails to start if you specify a port number that is already in use. After editing /etc/sysconfig/nfs, you need to restart the nfs-config service for the new values to take effect in Red Hat Enterprise Linux 7.2 and prior by running:
#  systemctl restart nfs-config
Then, restart the NFS server:
#  systemctl restart nfs-server
Run rpcinfo -p to confirm the changes have taken effect.

Note

To allow NFSv4.0 callbacks to pass through firewalls set /proc/sys/fs/nfs/nfs_callback_tcpport and allow the server to connect to that port on the client.
This process is not needed for NFSv4.1 or higher, and the other ports for mountd, statd, and lockd are not required in a pure NFSv4 environment.
8.6.3.1. Discovering NFS exports
There are two ways to discover which file systems an NFS server exports.
  • On any server that supports NFSv3, use the showmount command:
    $ showmount -e myserver
    Export list for mysever
    /exports/foo
    /exports/bar
  • On any server that supports NFSv4, mount the root directory and look around.
    # mount myserver:/ /mnt/
    # cd /mnt/
    exports
    # ls exports
    foo
    bar
On servers that support both NFSv4 and NFSv3, both methods work and give the same results.

Note

Before Red Hat Enterprise Linux 6 on older NFS servers, depending on how they are configured, it is possible to export filesystems to NFSv4 clients at different paths. Because these servers do not enable NFSv4 by default, this should not be a problem.

8.6.4. Accessing RPC Quota through a Firewall

If you export a file system that uses disk quotas, you can use the quota Remote Procedure Call (RPC) service to provide disk quota data to NFS clients.

Procedure 8.1. Making RPC Quota Accessible Behind a Firewall

  1. To enable the rpc-rquotad service, use the following command:
    # systemctl enable rpc-rquotad 
  2. To start the rpc-rquotad service, use the following command:
    # systemctl start rpc-rquotad 
    Note that rpc-rquotad is, if enabled, started automatically after starting the nfs-server service.
  3. To make the quota RPC service accessible behind a firewall, UDP or TCP port 875 need to be open. The default port number is defined in the /etc/services file.
    You can override the default port number by appending -p port-number to the RPCRQUOTADOPTS variable in the /etc/sysconfig/rpc-rquotad file.
  4. Restart rpc-rquotad for changes in the /etc/sysconfig/rpc-rquotad file to take effect:
    # systemctl restart rpc-rquotad

Setting Quotas from Remote Hosts

By default, quotas can only be read by remote hosts. To allow setting quotas, append the -S option to the RPCRQUOTADOPTS variable in the /etc/sysconfig/rpc-rquotad file.
Restart rpc-rquotad for changes in the /etc/sysconfig/rpc-rquotad file to take effect:
# systemctl restart rpc-rquotad

8.6.5. Hostname Formats

The host(s) can be in the following forms:
Single machine
A fully-qualified domain name (that can be resolved by the server), hostname (that can be resolved by the server), or an IP address.
Series of machines specified with wildcards
Use the * or ? character to specify a string match. Wildcards are not to be used with IP addresses; however, they may accidentally work if reverse DNS lookups fail. When specifying wildcards in fully qualified domain names, dots (.) are not included in the wildcard. For example, *.example.com includes one.example.com but does not include one.two.example.com.
IP networks
Use a.b.c.d/z, where a.b.c.d is the network and z is the number of bits in the netmask (for example 192.168.0.0/24). Another acceptable format is a.b.c.d/netmask, where a.b.c.d is the network and netmask is the netmask (for example, 192.168.100.8/255.255.255.0).
Netgroups
Use the format @group-name, where group-name is the NIS netgroup name.

8.6.6. Enabling NFS over RDMA (NFSoRDMA)

The remote direct memory access (RDMA) service works automatically in Red Hat Enterprise Linux 7 if there is RDMA-capable hardware present.
To enable NFS over RDMA:
  1. Install the rdma and rdma-core packages.
    The /etc/rdma/rdma.conf file contains a line that sets XPRTRDMA_LOAD=yes by default, which requests the rdma service to load the NFSoRDMA client module.
  2. To enable automatic loading of NFSoRDMA server modules, add SVCRDMA_LOAD=yes on a new line in /etc/rdma/rdma.conf.
    RPCNFSDARGS="--rdma=20049" in the /etc/sysconfig/nfs file specifies the port number on which the NFSoRDMA service listens for clients. RFC 5667 specifies that servers must listen on port 20049 when providing NFSv4 services over RDMA.
  3. Restart the nfs service after editing the /etc/rdma/rdma.conf file:
    # systemctl restart nfs
    Note that with earlier kernel versions, a system reboot is needed after editing /etc/rdma/rdma.conf for the changes to take effect.

8.6.7. Configuring an NFSv4-only Server

By default, the NFS server supports NFSv2, NFSv3, and NFSv4 connections in Red Hat Enterprise Linux 7. However, you can also configure NFS to support only NFS version 4.0 and later. This minimizes the number of open ports and running services on the system, because NFSv4 does not require the rpcbind service to listen on the network.
When your NFS server is configured as NFSv4-only, clients attempting to mount shares using NFSv2 or NFSv3 fail with an error like the following:
Requested NFS version or transport protocol is not supported.

Procedure 8.2. Configuring an NFSv4-only Server

To configure your NFS server to support only NFS version 4.0 and later:
  1. Disable NFSv2, NFSv3, and UDP by adding the following line to the /etc/sysconfig/nfs configuration file:
    RPCNFSDARGS="-N 2 -N 3 -U"
    
  2. Optionally, disable listening for the RPCBIND, MOUNT, and NSM protocol calls, which are not necessary in the NFSv4-only case.
    The effects of disabling these options are:
    • Clients that attempt to mount shares from your server using NFSv2 or NFSv3 become unresponsive.
    • The NFS server itself is unable to mount NFSv2 and NFSv3 file systems.
    To disable these options:
    • Add the following to the /etc/sysconfig/nfs file:
      RPCMOUNTDOPTS="-N 2 -N 3"
      
    • Disable related services:
      # systemctl mask --now rpc-statd.service rpcbind.service rpcbind.socket
  3. Restart the NFS server:
    # systemctl restart nfs
    The changes take effect as soon as you start or restart the NFS server.
Verifying the NFSv4-only Configuration
You can verify that your NFS server is configured in the NFSv4-only mode by using the netstat utility.
  • The following is an example netstat output on an NFSv4-only server; listening for RPCBIND, MOUNT, and NSM is also disabled. Here, nfs is the only listening NFS service:
    # netstat -ltu
    
    Active Internet connections (only servers)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State      
    tcp        0      0 0.0.0.0:nfs             0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN     
    tcp        0      0 localhost:smtp          0.0.0.0:*               LISTEN     
    tcp6       0      0 [::]:nfs                [::]:*                  LISTEN     
    tcp6       0      0 [::]:12432              [::]:*                  LISTEN     
    tcp6       0      0 [::]:12434              [::]:*                  LISTEN     
    tcp6       0      0 localhost:7092          [::]:*                  LISTEN     
    tcp6       0      0 [::]:ssh                [::]:*                  LISTEN     
    udp        0      0 localhost:323           0.0.0.0:*                          
    udp        0      0 0.0.0.0:bootpc          0.0.0.0:*                          
    udp6       0      0 localhost:323           [::]:*
    
  • In comparison, the netstat output before configuring an NFSv4-only server includes the sunrpc and mountd services:
    # netstat -ltu
    
    Active Internet connections (only servers)
    Proto Recv-Q Send-Q Local Address           Foreign Address         State      
    tcp        0      0 0.0.0.0:nfs             0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:36069           0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:52364           0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:sunrpc          0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:mountd          0.0.0.0:*               LISTEN     
    tcp        0      0 0.0.0.0:ssh             0.0.0.0:*               LISTEN     
    tcp        0      0 localhost:smtp          0.0.0.0:*               LISTEN     
    tcp6       0      0 [::]:34941              [::]:*                  LISTEN     
    tcp6       0      0 [::]:nfs                [::]:*                  LISTEN     
    tcp6       0      0 [::]:sunrpc             [::]:*                  LISTEN     
    tcp6       0      0 [::]:mountd             [::]:*                  LISTEN     
    tcp6       0      0 [::]:12432              [::]:*                  LISTEN     
    tcp6       0      0 [::]:56881              [::]:*                  LISTEN     
    tcp6       0      0 [::]:12434              [::]:*                  LISTEN     
    tcp6       0      0 localhost:7092          [::]:*                  LISTEN     
    tcp6       0      0 [::]:ssh                [::]:*                  LISTEN     
    udp        0      0 localhost:323           0.0.0.0:*                          
    udp        0      0 0.0.0.0:37190           0.0.0.0:*                          
    udp        0      0 0.0.0.0:876             0.0.0.0:*                          
    udp        0      0 localhost:877           0.0.0.0:*                          
    udp        0      0 0.0.0.0:mountd          0.0.0.0:*                          
    udp        0      0 0.0.0.0:38588           0.0.0.0:*                          
    udp        0      0 0.0.0.0:nfs             0.0.0.0:*                          
    udp        0      0 0.0.0.0:bootpc          0.0.0.0:*                          
    udp        0      0 0.0.0.0:sunrpc          0.0.0.0:*                          
    udp6       0      0 localhost:323           [::]:*                             
    udp6       0      0 [::]:57683              [::]:*                             
    udp6       0      0 [::]:876                [::]:*                             
    udp6       0      0 [::]:mountd             [::]:*                             
    udp6       0      0 [::]:40874              [::]:*                             
    udp6       0      0 [::]:nfs                [::]:*                             
    udp6       0      0 [::]:sunrpc             [::]:*
    

8.7. Securing NFS

NFS is suitable for transparent sharing of entire file systems with a large number of known hosts. However, with ease-of-use comes a variety of potential security problems. To minimize NFS security risks and protect data on the server, consider the following sections when exporting NFS file systems on a server or mounting them on a client.

8.7.1. NFS Security with AUTH_SYS and Export Controls

Traditionally, NFS has given two options in order to control access to exported files.
First, the server restricts which hosts are allowed to mount which file systems either by IP address or by host name.
Second, the server enforces file system permissions for users on NFS clients in the same way it does local users. Traditionally it does this using AUTH_SYS (also called AUTH_UNIX) which relies on the client to state the UID and GID's of the user. Be aware that this means a malicious or misconfigured client can easily get this wrong and allow a user access to files that it should not.
To limit the potential risks, administrators often allow read-only access or squash user permissions to a common user and group ID. Unfortunately, these solutions prevent the NFS share from being used in the way it was originally intended.
Additionally, if an attacker gains control of the DNS server used by the system exporting the NFS file system, the system associated with a particular hostname or fully qualified domain name can be pointed to an unauthorized machine. At this point, the unauthorized machine is the system permitted to mount the NFS share, since no username or password information is exchanged to provide additional security for the NFS mount.
Wildcards should be used sparingly when exporting directories through NFS, as it is possible for the scope of the wildcard to encompass more systems than intended.
It is also possible to restrict access to the rpcbind[1] service with TCP wrappers. Creating rules with iptables can also limit access to ports used by rpcbind, rpc.mountd, and rpc.nfsd.
For more information on securing NFS and rpcbind, refer to man iptables.

8.7.2. NFS Security with AUTH_GSS

NFSv4 revolutionized NFS security by mandating the implementation of RPCSEC_GSS and the Kerberos version 5 GSS-API mechanism. However, RPCSEC_GSS and the Kerberos mechanism are also available for all versions of NFS. In FIPS mode, only FIPS-approved algorithms can be used.
Unlike AUTH_SYS, with the RPCSEC_GSS Kerberos mechanism, the server does not depend on the client to correctly represent which user is accessing the file. Instead, cryptography is used to authenticate users to the server, which prevents a malicious client from impersonating a user without having that user's Kerberos credentials. Using the RPCSEC_GSS Kerberos mechanism is the most straightforward way to secure mounts because after configuring Kerberos, no additional setup is needed.

Configuring Kerberos

Before configuring an NFSv4 Kerberos-aware server, you need to install and configure a Kerberos Key Distribution Centre (KDC). Kerberos is a network authentication system that allows clients and servers to authenticate to each other by using symmetric encryption and a trusted third party, the KDC. Red Hat recommends using Identity Management (IdM) for setting up Kerberos.

Procedure 8.3. Configuring an NFS Server and Client for IdM to Use RPCSEC_GSS

    • Create the nfs/hostname.domain@REALM principal on the NFS server side.
    • Create the host/hostname.domain@REALM principal on both the server and the client side.

      Note

      The hostname must be identical to the NFS server hostname.
    • Add the corresponding keys to keytabs for the client and server.
  1. On the server side, use the sec= option to enable the wanted security flavors. To enable all security flavors as well as non-cryptographic mounts:
    /export *(sec=sys:krb5:krb5i:krb5p)
    
    Valid security flavors to use with the sec= option are:
    • sys: no cryptographic protection, the default
    • krb5: authentication only
    • krb5i: integrity protection
    • krb5p: privacy protection
  2. On the client side, add sec=krb5 (or sec=krb5i, or sec=krb5p, depending on the setup) to the mount options:
    # mount -o sec=krb5 server:/export /mnt
    
    For information on how to configure a NFS client, see the Setting up a Kerberos-aware NFS Client section in the Red Hat Enterprise Linux 7 Linux Domain Identity, Authentication, and Policy Guide.
Additional Resources
8.7.2.1. NFS Security with NFSv4
NFSv4 includes ACL support based on the Microsoft Windows NT model, not the POSIX model, because of the Microsoft Windows NT model's features and wide deployment.
Another important security feature of NFSv4 is the removal of the use of the MOUNT protocol for mounting file systems. The MOUNT protocol presented a security risk because of the way the protocol processed file handles.

8.7.3. File Permissions

Once the NFS file system is mounted as either read or read and write by a remote host, the only protection each shared file has is its permissions. If two users that share the same user ID value mount the same NFS file system, they can modify each others' files. Additionally, anyone logged in as root on the client system can use the su - command to access any files with the NFS share.
By default, access control lists (ACLs) are supported by NFS under Red Hat Enterprise Linux. Red Hat recommends that this feature is kept enabled.
By default, NFS uses root squashing when exporting a file system. This sets the user ID of anyone accessing the NFS share as the root user on their local machine to nobody. Root squashing is controlled by the default option root_squash; for more information about this option, refer to Section 8.6.1, “The /etc/exports Configuration File”. If possible, never disable root squashing.
When exporting an NFS share as read-only, consider using the all_squash option. This option makes every user accessing the exported file system take the user ID of the nfsnobody user.

8.8. NFS and rpcbind

Note

The following section only applies to NFSv3 implementations that require the rpcbind service for backward compatibility.
For information on how to configure an NFSv4-only server, which does not need rpcbind, see Section 8.6.7, “Configuring an NFSv4-only Server”.
The rpcbind[1] utility maps RPC services to the ports on which they listen. RPC processes notify rpcbind when they start, registering the ports they are listening on and the RPC program numbers they expect to serve. The client system then contacts rpcbind on the server with a particular RPC program number. The rpcbind service redirects the client to the proper port number so it can communicate with the requested service.
Because RPC-based services rely on rpcbind to make all connections with incoming client requests, rpcbind must be available before any of these services start.
The rpcbind service uses TCP wrappers for access control, and access control rules for rpcbind affect all RPC-based services. Alternatively, it is possible to specify access control rules for each of the NFS RPC daemons. The man pages for rpc.mountd and rpc.statd contain information regarding the precise syntax for these rules.

8.8.1. Troubleshooting NFS and rpcbind

Because rpcbind[1] provides coordination between RPC services and the port numbers used to communicate with them, it is useful to view the status of current RPC services using rpcbind when troubleshooting. The rpcinfo command shows each RPC-based service with port numbers, an RPC program number, a version number, and an IP protocol type (TCP or UDP).
To make sure the proper NFS RPC-based services are enabled for rpcbind, use the following command:
# rpcinfo -p

Example 8.7. rpcinfo -p command output

The following is sample output from this command:
program vers proto  port service
      100021    1   udp  32774  nlockmgr
      100021    3   udp  32774  nlockmgr
      100021    4   udp  32774  nlockmgr
      100021    1   tcp  34437  nlockmgr
      100021    3   tcp  34437  nlockmgr
      100021    4   tcp  34437  nlockmgr
      100011    1   udp    819  rquotad
      100011    2   udp    819  rquotad
      100011    1   tcp    822  rquotad
      100011    2   tcp    822  rquotad
      100003    2   udp   2049  nfs
      100003    3   udp   2049  nfs
      100003    2   tcp   2049  nfs
      100003    3   tcp   2049  nfs
      100005    1   udp    836  mountd
      100005    1   tcp    839  mountd
      100005    2   udp    836  mountd
      100005    2   tcp    839  mountd
      100005    3   udp    836  mountd
      100005    3   tcp    839  mountd
If one of the NFS services does not start up correctly, rpcbind will be unable to map RPC requests from clients for that service to the correct port. In many cases, if NFS is not present in rpcinfo output, restarting NFS causes the service to correctly register with rpcbind and begin working.
For more information and a list of options on rpcinfo, see its man page.

8.9. pNFS

Support for Parallel NFS (pNFS) as part of the NFS v4.1 standard is available as of Red Hat Enterprise Linux 6.4. The pNFS architecture improves the scalability of NFS, with possible improvements to performance. That is, when a server implements pNFS as well, a client is able to access data through multiple servers concurrently. It supports three storage protocols or layouts: files, objects, and blocks.

Note

The protocol allows for three possible pNFS layout types: files, objects, and blocks. While the Red Hat Enterprise Linux 6.4 client only supported the files layout type, Red Hat Enterprise Linux 7 supports the files layout type, with objects and blocks layout types being included as a technology preview.

pNFS Flex Files

Flexible Files is a new layout for pNFS that enables the aggregation of standalone NFSv3 and NFSv4 servers into a scale out name space. The Flex Files feature is part of the NFSv4.2 standard as described in the RFC 7862 specification.
Red Hat Enterprise Linux can mount NFS shares from Flex Files servers since Red Hat Enterprise Linux 7.4.

Mounting pNFS Shares

  • To enable pNFS functionality, mount shares from a pNFS-enabled server with NFS version 4.1 or later:
    # mount -t nfs -o v4.1 server:/remote-export /local-directory
    After the server is pNFS-enabled, the nfs_layout_nfsv41_files kernel is automatically loaded on the first mount. The mount entry in the output should contain minorversion=1. Use the following command to verify the module was loaded:
    $ lsmod | grep nfs_layout_nfsv41_files
  • To mount an NFS share with the Flex Files feature from a server that supports Flex Files, use NFS version 4.2 or later:
    # mount -t nfs -o v4.2 server:/remote-export /local-directory
    Verify that the nfs_layout_flexfiles module has been loaded:
    $ lsmod | grep nfs_layout_flexfiles

Additional Resources

For more information on pNFS, refer to: http://www.pnfs.com.

8.10. Enabling pNFS SCSI Layouts in NFS

You can configure the NFS server and client to use the pNFS SCSI layout for accessing data. pNFS SCSI is beneficial in use cases that involve longer-duration single-client access to a file.

Prerequisites

  • Both the client and the server must be able to send SCSI commands to the same block device. That is, the block device must be on a shared SCSI bus.
  • The block device must contain an XFS file system.
  • The SCSI device must support SCSI Persistent Reservations as described in the SCSI-3 Primary Commands specification.

8.10.1. pNFS SCSI Layouts

The SCSI layout builds on the work of pNFS block layouts. The layout is defined across SCSI devices. It contains a sequential series of fixed-size blocks as logical units (LUs) that must be capable of supporting SCSI persistent reservations. The LU devices are identified by their SCSI device identification.
pNFS SCSI performs well in use cases that involve longer-duration single-client access to a file. An example might be a mail server or a virtual machine housing a cluster.
Operations Between the Client and the Server
When an NFS client reads from a file or writes to it, the client performs a LAYOUTGET operation. The server responds with the location of the file on the SCSI device. The client might need to perform an additional operation of GETDEVICEINFO to determine which SCSI device to use. If these operations work correctly, the client can issue I/O requests directly to the SCSI device instead of sending READ and WRITE operations to the server.
Errors or contention between clients might cause the server to recall layouts or not issue them to the clients. In those cases, the clients fall back to issuing READ and WRITE operations to the server instead of sending I/O requests directly to the SCSI device.
Device Reservations
pNFS SCSI handles fencing through the assignment of reservations. Before the server issues layouts to clients, it reserves the SCSI device to ensure that only registered clients may access the device. If a client can issue commands to that SCSI device but is not registered with the device, many operations from the client on that device fail. For example, the blkid command on the client fails to show the UUID of the XFS file system if the server has not given a layout for that device to the client.
The server does not remove its own persistent reservation. This protects the data within the file system on the device across restarts of clients and servers. In order to repurpose the SCSI device, you might need to manually remove the persistent reservation on the NFS server.

8.10.2. Checking for a SCSI Device Compatible with pNFS

This procedure checks if a SCSI device supports the pNFS SCSI layout.
Prerequisites
  • Install the sg3_utils package:
    # yum install sg3_utils

Procedure 8.4. Checking for a SCSI Device Compatible with pNFS

  • On both the server and client, check for the proper SCSI device support:
    # sg_persist --in --report-capabilities --verbose path-to-scsi-device
    Ensure that the Persist Through Power Loss Active (PTPL_A) bit is set.

    Example 8.8.  A SCSI device that supports pNFS SCSI

    The following is an example of sg_persist output for a SCSI device that supports pNFS SCSI. The PTPL_A bit reports 1.
        inquiry cdb: 12 00 00 00 24 00
        Persistent Reservation In cmd: 5e 02 00 00 00 00 00 20 00 00
      LIO-ORG   block11           4.0
      Peripheral device type: disk
    Report capabilities response:
      Compatible Reservation Handling(CRH): 1
      Specify Initiator Ports Capable(SIP_C): 1
      All Target Ports Capable(ATP_C): 1
      Persist Through Power Loss Capable(PTPL_C): 1
      Type Mask Valid(TMV): 1
      Allow Commands: 1
      Persist Through Power Loss Active(PTPL_A): 1
        Support indicated in Type mask:
          Write Exclusive, all registrants: 1
          Exclusive Access, registrants only: 1
          Write Exclusive, registrants only: 1
          Exclusive Access: 1
          Write Exclusive: 1
          Exclusive Access, all registrants: 1
    
Additional Resources
  • The sg_persist(8) man page

8.10.3. Setting up pNFS SCSI on the Server

This procedure configures an NFS server to export a pNFS SCSI layout.

Procedure 8.5. Setting up pNFS SCSI on the Server

  1. On the server, mount the XFS file system created on the SCSI device.
  2. Configure the NFS server to export NFS version 4.1 or higher. Set the following option in the [nfsd] section of the /etc/nfs.conf file:
    [nfsd]
    
    vers4.1=y
    
  3. Configure the NFS server to export the XFS file system over NFS with the pnfs option:

    Example 8.9. An Entry in /etc/exports to Export pNFS SCSI

    The following entry in the /etc/exports configuration file exports the file system mounted at /exported/directory/ to the allowed.example.com client as a pNFS SCSI layout:
    /exported/directory allowed.example.com(pnfs)
Additional Resources

8.10.4. Setting up pNFS SCSI on the Client

This procedure configures an NFS client to mount a pNFS SCSI layout.
Prerequisites

Procedure 8.6. Setting up pNFS SCSI on the Client

  • On the client, mount the exported XFS file system using NFS version 4.1 or higher:
    # mount -t nfs -o nfsvers=4.1 host:/remote/export /local/directory
    Do not mount the XFS file system directly without NFS.
Additional Resources

8.10.5. Releasing the pNFS SCSI Reservation on the Server

This procedure releases the persistent reservation that an NFS server holds on a SCSI device. This enables you to repurpose the SCSI device when you no longer need to export pNFS SCSI.
You must remove the reservation from the server. It cannot be removed from a different IT Nexus.
Prerequisites
  • Install the sg3_utils package:
    # yum install sg3_utils

Procedure 8.7. Releasing the pNFS SCSI Reservation on the Server

  1. Query an existing reservation on the server:
    # sg_persist --read-reservation path-to-scsi-device

    Example 8.10. Querying a Reservation on /dev/sda

    # sg_persist --read-reservation /dev/sda
    
      LIO-ORG   block_1           4.0
      Peripheral device type: disk
      PR generation=0x8, Reservation follows:
        Key=0x100000000000000
        scope: LU_SCOPE,  type: Exclusive Access, registrants only
    
  2. Remove the existing registration on the server:
    # sg_persist --out \
                 --release \
                 --param-rk=reservation-key \
                 --prout-type=6 \
                 path-to-scsi-device

    Example 8.11. Removing a Reservation on /dev/sda

    # sg_persist --out \
                 --release \
                 --param-rk=0x100000000000000 \
                 --prout-type=6 \
                 /dev/sda
    
      LIO-ORG   block_1           4.0
      Peripheral device type: disk
    
Additional Resources
  • The sg_persist(8) man page

8.10.6. Monitoring pNFS SCSI Layouts Functionality

You can monitor if the pNFS client and server exchange proper pNFS SCSI operations or if they fall back on regular NFS operations.
Prerequisites
  • A pNFS SCSI client and server are configured.
8.10.6.1. Checking pNFS SCSI Operations from the Server Using nfsstat
This procedure uses the nfsstat utility to monitor pNFS SCSI operations from the server.

Procedure 8.8. Checking pNFS SCSI Operations from the Server Using nfsstat

  1. Monitor the operations serviced from the server:
    # watch --differences \
            "nfsstat --server | egrep --after-context=1 read\|write\|layout"
    
    Every 2.0s: nfsstat --server | egrep --after-context=1 read\|write\|layout
    
    putrootfh    read         readdir      readlink     remove	 rename
    2         0% 0         0% 1         0% 0         0% 0         0% 0         0%
    --
    setcltidconf verify	  write        rellockowner bc_ctl	 bind_conn
    0         0% 0         0% 0         0% 0         0% 0         0% 0         0%
    --
    getdevlist   layoutcommit layoutget    layoutreturn secinfononam sequence
    0         0% 29        1% 49        1% 5         0% 0         0% 2435     86%
    
  2. The client and server use pNFS SCSI operations when:
    • The layoutget, layoutreturn, and layoutcommit counters increment. This means that the server is serving layouts.
    • The server read and write counters do not increment. This means that the clients are performing I/O requests directly to the SCSI devices.
8.10.6.2. Checking pNFS SCSI Operations from the Client Using mountstats
This procedure uses the /proc/self/mountstats file to monitor pNFS SCSI operations from the client.

Procedure 8.9. Checking pNFS SCSI Operations from the Client Using mountstats

  1. List the per-mount operation counters:
    # cat /proc/self/mountstats \
          | awk /scsi_lun_0/,/^$/ \
          | egrep device\|READ\|WRITE\|LAYOUT
    
    device 192.168.122.73:/exports/scsi_lun_0 mounted on /mnt/rhel7/scsi_lun_0 with fstype nfs4 statvers=1.1
        nfsv4:  bm0=0xfdffbfff,bm1=0x40f9be3e,bm2=0x803,acl=0x3,sessions,pnfs=LAYOUT_SCSI
                READ: 0 0 0 0 0 0 0 0
               WRITE: 0 0 0 0 0 0 0 0
            READLINK: 0 0 0 0 0 0 0 0
             READDIR: 0 0 0 0 0 0 0 0
           LAYOUTGET: 49 49 0 11172 9604 2 19448 19454
        LAYOUTCOMMIT: 28 28 0 7776 4808 0 24719 24722
        LAYOUTRETURN: 0 0 0 0 0 0 0 0
         LAYOUTSTATS: 0 0 0 0 0 0 0 0
    
  2. In the results:
    • The LAYOUT statistics indicate requests where the client and server use pNFS SCSI operations.
    • The READ and WRITE statistics indicate requests where the client and server fall back to NFS operations.

8.11. NFS References

Administering an NFS server can be a challenge. Many options, including quite a few not mentioned in this chapter, are available for exporting or mounting NFS shares. For more information, see the following sources:

Installed Documentation

  • man mount — Contains a comprehensive look at mount options for both NFS server and client configurations.
  • man fstab — Provides detail for the format of the /etc/fstab file used to mount file systems at boot-time.
  • man nfs — Provides details on NFS-specific file system export and mount options.
  • man exports — Shows common options used in the /etc/exports file when exporting NFS file systems.

Useful Websites



[1] The rpcbind service replaces portmap, which was used in previous versions of Red Hat Enterprise Linux to map RPC program numbers to IP address port number combinations. For more information, refer to Section 8.1.1, “Required Services”.

Chapter 9. Server Message Block (SMB)

The Server Message Block (SMB) protocol implements an application-layer network protocol used to access resources on a server, such as file shares and shared printers. On Microsoft Windows, SMB is implemented by default. If you run Red Hat Enterprise Linux, use Samba to provide SMB shares and the cifs-utils utility to mount SMB shares from a remote server.

Note

In the context of SMB, you sometimes read about the Common Internet File System (CIFS) protocol, which is a dialect of SMB. Both the SMB and CIFS protocol are supported and the kernel module and utilities involved in mounting SMB and CIFS shares both use the name cifs.

9.1. Providing SMB Shares

See the Samba section in the Red Hat System Administrator's Guide.

9.2. Mounting an SMB Share

On Red Hat Enterprise Linux, the cifs.ko file system module of the kernel provides support for the SMB protocol. However, to mount and work with SMB shares, you must also install the cifs-utils package:
# yum install cifs-utils
The cifs-utils package provides utilities to:
  • Mount SMB and CIFS shares
  • Manage NT Lan Manager (NTLM) credentials in the kernel's keyring
  • Set and display Access Control Lists (ACL) in a security descriptor on SMB and CIFS shares

9.2.1. Supported SMB Protocol Versions

The cifs.ko kernel module supports the following SMB protocol versions:
  • SMB 1
  • SMB 2.0
  • SMB 2.1
  • SMB 3.0

Note

Depending on the protocol version, not all SMB features are implemented.
9.2.1.1. UNIX Extensions Support
Samba uses the CAP_UNIX capability bit in the SMB protocol to provide the UNIX extensions feature. These extensions are also supported by the cifs.ko kernel module. However, both Samba and the kernel module support UNIX extensions only in the SMB 1 protocol.
To use UNIX extensions:
  1. Set the server min protocol option in the [global] section in the /etc/samba/smb.conf file to NT1. This is the default on Samba servers.
  2. Mount the share using the SMB 1 protocol by providing the -o vers=1.0 option to the mount command. For example:
    mount -t cifs -o vers=1.0,username=user_name //server_name/share_name /mnt/
    By default, the kernel module uses SMB 2 or the highest later protocol version supported by the server. Passing the -o vers=1.0 option to the mount command forces that the kernel module uses the SMB 1 protocol that is required for using UNIX extensions.
To verify if UNIX extensions are enabled, display the options of the mounted share:
# mount
...
//server/share on /mnt type cifs (...,unix,...)
If the unix entry is displayed in the list of mount options, UNIX extensions are enabled.

9.2.2. Manually Mounting an SMB Share

To manually mount an SMB share, use the mount utility with the -t cifs parameter:
# mount -t cifs -o username=user_name //server_name/share_name /mnt/
Password for user_name@//server_name/share_name:  ********
In the -o options parameter, you can specify options that will be used to mount the share. For details, see Section 9.2.6, “Frequently Used Mount Options” and the OPTIONS section in the mount.cifs(8) man page.

Example 9.1. Mounting a Share Using an Encrypted SMB 3.0 Connection

To mount the \\server\example\ share as the DOMAIN\Administrator user over an encrypted SMB 3.0 connection into the /mnt/ directory:
# mount -t cifs -o username=DOMAIN\Administrator,seal,vers=3.0 //server/example /mnt/
Password for user_name@//server_name/share_name:  ********

9.2.3. Mounting an SMB Share Automatically When the System Boots

To mount an SMB share automatically when the system boots, add an entry for the share to the /etc/fstab file. For example:
//server_name/share_name  /mnt  cifs  credentials=/root/smb.cred  0 0

Important

To enable the system to mount a share automatically, you must store the user name, password, and domain name in a credentials file. For details, see Section 9.2.4, “Authenticating To an SMB Share Using a Credentials File”.
In the fourth field of the /etc/fstab file, specify mount options, such as the path to the credentials file. For details, see Section 9.2.6, “Frequently Used Mount Options” and the OPTIONS section in the mount.cifs(8) man page.
To verify that the share mounts successfully, enter:
# mount /mnt/

9.2.4. Authenticating To an SMB Share Using a Credentials File

In certain situations, administrators want to mount a share without entering the user name and password. To implement this, create a credentials file. For example:

Procedure 9.1. Creating a Credentials File

  1. Create a file, such as ~/smb.cred, and specify the user name, password, and domain name that file:
    username=user_name
    password=password
    domain=domain_name
  2. Set the permissions to only allow the owner to access the file:
    # chown user_name ~/smb.cred
    # chmod 600 ~/smb.cred
You can now pass the credentials=file_name mount option to the mount utility or use it in the /etc/fstab file to mount the share without being prompted for the user name and password.

9.2.5. Performing a Multi-user SMB Mount

The credentials you provide to mount a share determine the access permissions on the mount point by default. For example, if you use the DOMAIN\example user when you mount a share, all operations on the share will be executed as this user, regardless which local user performs the operation.
However, in certain situations, the administrator wants to mount a share automatically when the system boots, but users should perform actions on the share's content using their own credentials. The multiuser mount options lets you configure this scenario.

Important

To use multiuser, you must additionally set the sec=security_type mount option to a security type which supports providing credentials in a non-interactive way, such as krb5 or the ntlmssp option with a credentials file. See the section called “Accessing a Share as a User”.
The root user mounts the share using the multiuser option and an account that has minimal access to the contents of the share. Regular users can then provide their user name and password to the current session's kernel keyring using the cifscreds utility. If the user accesses the content of the mounted share, the kernel uses the credentials from the kernel keyring instead of the one initially used to mount the share.
Mounting a Share with the multiuser Option
To mount a share automatically with the multiuser option when the system boots:

Procedure 9.2. Creating an /etc/fstab File Entry with the multiuser Option

  1. Create the entry for the share in the /etc/fstab file. For example:
    //server_name/share_name  /mnt  cifs  multiuser,sec=ntlmssp,credentials=/root/smb.cred  0 0
    
  2. Mount the share:
    # mount /mnt/
If you do not want to mount the share automatically when the system boots, mount it manually by passing -o multiuser,sec=security_type to the mount command. For details about mounting an SMB share manually, see Section 9.2.2, “Manually Mounting an SMB Share”.
Verifying if an SMB Share is Mounted with the multiuser Option
To verify if a share is mounted with the multiuser option:
# mount
...
//server_name/share_name on /mnt type cifs (sec=ntlmssp,multiuser,...)
Accessing a Share as a User
If an SMB share is mounted with the multiuser option, users can provide their credentials for the server to the kernel's keyring:
# cifscreds add -u SMB_user_name server_name
Password: ********
Now, when the user performs operations in the directory that contains the mounted SMB share, the server applies the file system permissions for this user, instead of the one initially used when the share was mounted.

Note

Multiple users can perform operations using their own credentials on the mounted share at the same time.

9.2.6. Frequently Used Mount Options

When you mount an SMB share, the mount options determine:
  • How the connection will be established with the server. For example, which SMB protocol version is used when connecting to the server.
  • How the share will be mounted into the local file system. For example, if the system overrides the remote file and directory permissions to enable multiple local users to access the content on the server.
To set multiple options in the fourth field of the /etc/fstab file or in the -o parameter of a mount command, separate them with commas. For example, see Procedure 9.2, “Creating an /etc/fstab File Entry with the multiuser Option”.
The following list gives an overview of frequently used mount options:
Table 9.1. Frequently Used Mount Options
Option Description
credentials=file_name Sets the path to the credentials file. See Section 9.2.4, “Authenticating To an SMB Share Using a Credentials File”.
dir_mode=mode Sets the directory mode if the server does not support CIFS UNIX extensions.
file_mode=mode Sets the file mode if the server does not support CIFS UNIX extensions.
password=password Sets the password used to authenticate to the SMB server. Alternatively, specify a credentials file using the credentials option.
seal Enables encryption support for connections using SMB 3.0 or a later protocol version. Therefore, use seal together with the vers mount option set to 3.0 or later. See Example 9.1, “Mounting a Share Using an Encrypted SMB 3.0 Connection”.
sec=security_mode
Sets the security mode, such as ntlmsspi, to enable NTLMv2 password hashing and enabled packet signing. For a list of supported values, see the option's description in the mount.cifs(8) man page.
If the server does not support the ntlmv2 security mode, use sec=ntlmssp, which is the default. For security reasons, do not use the insecure ntlm security mode.
username=user_name Sets the user name used to authenticate to the SMB server. Alternatively, specify a credentials file using the credentials option.
vers=SMB_protocol_version Sets the SMB protocol version used for the communication with the server.
For a complete list, see the OPTIONS section in the mount.cifs(8) man page.

Chapter 10. FS-Cache

FS-Cache is a persistent local cache that can be used by file systems to take data retrieved from over the network and cache it on local disk. This helps minimize network traffic for users accessing data from a file system mounted over the network (for example, NFS).
The following diagram is a high-level illustration of how FS-Cache works:
FS-Cache Overview

Figure 10.1. FS-Cache Overview

FS-Cache is designed to be as transparent as possible to the users and administrators of a system. Unlike cachefs on Solaris, FS-Cache allows a file system on a server to interact directly with a client's local cache without creating an overmounted file system. With NFS, a mount option instructs the client to mount the NFS share with FS-cache enabled.
FS-Cache does not alter the basic operation of a file system that works over the network - it merely provides that file system with a persistent place in which it can cache data. For instance, a client can still mount an NFS share whether or not FS-Cache is enabled. In addition, cached NFS can handle files that won't fit into the cache (whether individually or collectively) as files can be partially cached and do not have to be read completely up front. FS-Cache also hides all I/O errors that occur in the cache from the client file system driver.
To provide caching services, FS-Cache needs a cache back end. A cache back end is a storage driver configured to provide caching services (i.e. cachefiles). In this case, FS-Cache requires a mounted block-based file system that supports bmap and extended attributes (e.g. ext3) as its cache back end.
FS-Cache cannot arbitrarily cache any file system, whether through the network or otherwise: the shared file system's driver must be altered to allow interaction with FS-Cache, data storage/retrieval, and metadata setup and validation. FS-Cache needs indexing keys and coherency data from the cached file system to support persistence: indexing keys to match file system objects to cache objects, and coherency data to determine whether the cache objects are still valid.

Note

In Red Hat Enterprise Linux 7, the cachefilesd package is not installed by default and needs to be installed manually.

10.1. Performance Guarantee

FS-Cache does not guarantee increased performance, however it ensures consistent performance by avoiding network congestion. Using a cache back end incurs a performance penalty: for example, cached NFS shares add disk accesses to cross-network lookups. While FS-Cache tries to be as asynchronous as possible, there are synchronous paths (e.g. reads) where this isn't possible.
For example, using FS-Cache to cache an NFS share between two computers over an otherwise unladen GigE network will not demonstrate any performance improvements on file access. Rather, NFS requests would be satisfied faster from server memory rather than from local disk.
The use of FS-Cache, therefore, is a compromise between various factors. If FS-Cache is being used to cache NFS traffic, for instance, it may slow the client down a little, but massively reduce the network and server loading by satisfying read requests locally without consuming network bandwidth.

10.2. Setting up a Cache

Currently, Red Hat Enterprise Linux 7 only provides the cachefiles caching back end. The cachefilesd daemon initiates and manages cachefiles. The /etc/cachefilesd.conf file controls how cachefiles provides caching services.
The first setting to configure in a cache back end is which directory to use as a cache. To configure this, use the following parameter:
$ dir /path/to/cache
Typically, the cache back end directory is set in /etc/cachefilesd.conf as /var/cache/fscache, as in:
$ dir /var/cache/fscache
If you want to change the cache back end directory, the selinux context must be same as /var/cache/fscache:
# semanage fcontext -a -e /var/cache/fscache /path/to/cache
# restorecon -Rv /path/to/cache
Replace /path/to/cache with the directory name while setting up cache.

Note

If the given commands for setting selinux context did not work, use the following commands:
# semanage permissive -a cachefilesd_t
# semanage permissive -a cachefiles_kernel_t
FS-Cache will store the cache in the file system that hosts /path/to/cache. On a laptop, it is advisable to use the root file system (/) as the host file system, but for a desktop machine it would be more prudent to mount a disk partition specifically for the cache.
File systems that support functionalities required by FS-Cache cache back end include the Red Hat Enterprise Linux 7 implementations of the following file systems:
  • ext3 (with extended attributes enabled)
  • ext4
  • Btrfs
  • XFS
The host file system must support user-defined extended attributes; FS-Cache uses these attributes to store coherency maintenance information. To enable user-defined extended attributes for ext3 file systems (i.e. device), use:
# tune2fs -o user_xattr /dev/device
Alternatively, extended attributes for a file system can be enabled at mount time, as in:
# mount /dev/device /path/to/cache -o user_xattr
The cache back end works by maintaining a certain amount of free space on the partition hosting the cache. It grows and shrinks the cache in response to other elements of the system using up free space, making it safe to use on the root file system (for example, on a laptop). FS-Cache sets defaults on this behavior, which can be configured via cache cull limits. For more information about configuring cache cull limits, refer to Section 10.4, “Setting Cache Cull Limits”.
Once the configuration file is in place, start up the cachefilesd service:
# systemctl start cachefilesd
To configure cachefilesd to start at boot time, execute the following command as root:
# systemctl enable cachefilesd

10.3. Using the Cache with NFS

NFS will not use the cache unless explicitly instructed. To configure an NFS mount to use FS-Cache, include the -o fsc option to the mount command:
# mount nfs-share:/ /mount/point -o fsc
All access to files under /mount/point will go through the cache, unless the file is opened for direct I/O or writing. For more information, see Section 10.3.2, “Cache Limitations with NFS”. NFS indexes cache contents using NFS file handle, not the file name, which means hard-linked files share the cache correctly.
Caching is supported in version 2, 3, and 4 of NFS. However, each version uses different branches for caching.

10.3.1. Cache Sharing

There are several potential issues to do with NFS cache sharing. Because the cache is persistent, blocks of data in the cache are indexed on a sequence of four keys:
  • Level 1: Server details
  • Level 2: Some mount options; security type; FSID; uniquifier
  • Level 3: File Handle
  • Level 4: Page number in file
To avoid coherency management problems between superblocks, all NFS superblocks that wish to cache data have unique Level 2 keys. Normally, two NFS mounts with same source volume and options share a superblock, and thus share the caching, even if they mount different directories within that volume.

Example 10.1. Cache Sharing

Take the following two mount commands:
mount home0:/disk0/fred /home/fred -o fsc
mount home0:/disk0/jim /home/jim -o fsc
Here, /home/fred and /home/jim likely share the superblock as they have the same options, especially if they come from the same volume/partition on the NFS server (home0). Now, consider the next two subsequent mount commands:
mount home0:/disk0/fred /home/fred -o fsc,rsize=230
mount home0:/disk0/jim /home/jim -o fsc,rsize=231
In this case, /home/fred and /home/jim will not share the superblock as they have different network access parameters, which are part of the Level 2 key. The same goes for the following mount sequence:
mount home0:/disk0/fred /home/fred1 -o fsc,rsize=230
mount home0:/disk0/fred /home/fred2 -o fsc,rsize=231
Here, the contents of the two subtrees (/home/fred1 and /home/fred2) will be cached twice.
Another way to avoid superblock sharing is to suppress it explicitly with the nosharecache parameter. Using the same example:
mount home0:/disk0/fred /home/fred -o nosharecache,fsc
mount home0:/disk0/jim /home/jim -o nosharecache,fsc
However, in this case only one of the superblocks is permitted to use cache since there is nothing to distinguish the Level 2 keys of home0:/disk0/fred and home0:/disk0/jim. To address this, add a unique identifier on at least one of the mounts, i.e. fsc=unique-identifier. For example:
mount home0:/disk0/fred /home/fred -o nosharecache,fsc
mount home0:/disk0/jim /home/jim -o nosharecache,fsc=jim
Here, the unique identifier jim is added to the Level 2 key used in the cache for /home/jim.

10.3.2. Cache Limitations with NFS

  • Opening a file from a shared file system for direct I/O automatically bypasses the cache. This is because this type of access must be direct to the server.
  • Opening a file from a shared file system for writing will not work on NFS version 2 and 3. The protocols of these versions do not provide sufficient coherency management information for the client to detect a concurrent write to the same file from another client.
  • Opening a file from a shared file system for either direct I/O or writing flushes the cached copy of the file. FS-Cache will not cache the file again until it is no longer opened for direct I/O or writing.
  • Furthermore, this release of FS-Cache only caches regular NFS files. FS-Cache will not cache directories, symlinks, device files, FIFOs and sockets.

10.4. Setting Cache Cull Limits

The cachefilesd daemon works by caching remote data from shared file systems to free space on the disk. This could potentially consume all available free space, which could be bad if the disk also housed the root partition. To control this, cachefilesd tries to maintain a certain amount of free space by discarding old objects (i.e. accessed less recently) from the cache. This behavior is known as cache culling.
Cache culling is done on the basis of the percentage of blocks and the percentage of files available in the underlying file system. There are six limits controlled by settings in /etc/cachefilesd.conf:
brun N% (percentage of blocks) , frun N% (percentage of files)
If the amount of free space and the number of available files in the cache rises above both these limits, then culling is turned off.
bcull N% (percentage of blocks), fcull N% (percentage of files)
If the amount of available space or the number of files in the cache falls below either of these limits, then culling is started.
bstop N% (percentage of blocks), fstop N% (percentage of files)
If the amount of available space or the number of available files in the cache falls below either of these limits, then no further allocation of disk space or files is permitted until culling has raised things above these limits again.
The default value of N for each setting is as follows:
  • brun/frun - 10%
  • bcull/fcull - 7%
  • bstop/fstop - 3%
When configuring these settings, the following must hold true:
  • 0 ≤ bstop < bcull < brun < 100
  • 0 ≤ fstop < fcull < frun < 100
These are the percentages of available space and available files and do not appear as 100 minus the percentage displayed by the df program.

Important

Culling depends on both bxxx and fxxx pairs simultaneously; they can not be treated separately.

10.5. Statistical Information

FS-Cache also keeps track of general statistical information. To view this information, use:
# cat /proc/fs/fscache/stats
FS-Cache statistics includes information on decision points and object counters. For more information, see the following kernel document:
/usr/share/doc/kernel-doc-version/Documentation/filesystems/caching/fscache.txt

10.6. FS-Cache References

For more information on cachefilesd and how to configure it, see man cachefilesd and man cachefilesd.conf. The following kernel documents also provide additional information:
  • /usr/share/doc/cachefilesd-version-number/README
  • /usr/share/man/man5/cachefilesd.conf.5.gz
  • /usr/share/man/man8/cachefilesd.8.gz
For general information about FS-Cache, including details on its design constraints, available statistics, and capabilities, see the following kernel document: /usr/share/doc/kernel-doc-version/Documentation/filesystems/caching/fscache.txt

Part II. Storage Administration

The Storage Administration section starts with storage considerations for Red Hat Enterprise Linux 7. Instructions regarding partitions, logical volume management, and swap partitions follow this. Disk Quotas, RAID systems are next, followed by the functions of mount command, volume_key, and acls. SSD tuning, write barriers, I/O limits and diskless systems follow this. The large chapter of Online Storage is next, and finally device mapper multipathing and virtual storage to finish.

Chapter 11. Storage Considerations During Installation

Many storage device and file system settings can only be configured at install time. Other settings, such as file system type, can only be modified up to a certain point without requiring a reformat. As such, it is prudent that you plan your storage configuration accordingly before installing Red Hat Enterprise Linux 7.
This chapter discusses several considerations when planning a storage configuration for your system. For installation instructions (including storage configuration during installation), see the Installation Guide provided by Red Hat.
For information on what Red Hat officially supports with regards to size and storage limits, see the article http://www.redhat.com/resourcelibrary/articles/articles-red-hat-enterprise-linux-6-technology-capabilities-and-limits.

11.1. Special Considerations

This section enumerates several issues and factors to consider for specific storage configurations.

Separate Partitions for /home, /opt, /usr/local

If it is likely that you will upgrade your system in the future, place /home, /opt, and /usr/local on a separate device. This allows you to reformat the devices or file systems containing the operating system while preserving your user and application data.

DASD and zFCP Devices on IBM System Z

On the IBM System Z platform, DASD and zFCP devices are configured via the Channel Command Word (CCW) mechanism. CCW paths must be explicitly added to the system and then brought online. For DASD devices, this means listing the device numbers (or device number ranges) as the DASD= parameter at the boot command line or in a CMS configuration file.
For zFCP devices, you must list the device number, logical unit number (LUN), and world wide port name (WWPN). Once the zFCP device is initialized, it is mapped to a CCW path. The FCP_x= lines on the boot command line (or in a CMS configuration file) allow you to specify this information for the installer.

Encrypting Block Devices Using LUKS

Formatting a block device for encryption using LUKS/dm-crypt destroys any existing formatting on that device. As such, you should decide which devices to encrypt (if any) before the new system's storage configuration is activated as part of the installation process.

Stale BIOS RAID Metadata

Moving a disk from a system configured for firmware RAID without removing the RAID metadata from the disk can prevent Anaconda from correctly detecting the disk.

Warning

Removing/deleting RAID metadata from disk could potentially destroy any stored data. Red Hat recommends that you back up your data before proceeding.

Note

If you have created the RAID volume using dmraid, which is now deprecated, use the dmraid utility to delete it:
# dmraid -r -E /device/
For more information about managing RAID devices, see man dmraid and Chapter 18, Redundant Array of Independent Disks (RAID).

iSCSI Detection and Configuration

For plug and play detection of iSCSI drives, configure them in the firmware of an iBFT boot-capable network interface card (NIC). CHAP authentication of iSCSI targets is supported during installation. However, iSNS discovery is not supported during installation.

FCoE Detection and Configuration

For plug and play detection of Fibre Channel over Ethernet (FCoE) drives, configure them in the firmware of an EDD boot-capable NIC.

DASD

Direct-access storage devices (DASD) cannot be added or configured during installation. Such devices are specified in the CMS configuration file.

Block Devices with DIF/DIX Enabled

DIF/DIX is a hardware checksum feature provided by certain SCSI host bus adapters and block devices. When DIF/DIX is enabled, error occurs if the block device is used as a general-purpose block device. Buffered I/O or mmap(2)-based I/O will not work reliably, as there are no interlocks in the buffered write path to prevent buffered data from being overwritten after the DIF/DIX checksum has been calculated.
This causes the I/O to later fail with a checksum error. This problem is common to all block device (or file system-based) buffered I/O or mmap(2) I/O, so it is not possible to work around these errors caused by overwrites.
As such, block devices with DIF/DIX enabled should only be used with applications that use O_DIRECT. Such applications should use the raw block device. Alternatively, it is also safe to use the XFS file system on a DIF/DIX enabled block device, as long as only O_DIRECT I/O is issued through the file system. XFS is the only file system that does not fall back to buffered I/O when doing certain allocation operations.
The responsibility for ensuring that the I/O data does not change after the DIF/DIX checksum has been computed always lies with the application, so only applications designed for use with O_DIRECT I/O and DIF/DIX hardware should use DIF/DIX.

Chapter 12. File System Check

File systems may be checked for consistency, and optionally repaired, with file system-specific userspace tools. These tools are often referred to as fsck tools, where fsck is a shortened version of file system check.

Note

These file system checkers only guarantee metadata consistency across the file system; they have no awareness of the actual data contained within the file system and are not data recovery tools.
File system inconsistencies can occur for various reasons, including but not limited to hardware errors, storage administration errors, and software bugs.
Before modern metadata-journaling file systems became common, a file system check was required any time a system crashed or lost power. This was because a file system update could have been interrupted, leading to an inconsistent state. As a result, a file system check is traditionally run on each file system listed in /etc/fstab at boot-time. For journaling file systems, this is usually a very short operation, because the file system's metadata journaling ensures consistency even after a crash.
However, there are times when a file system inconsistency or corruption may occur, even for journaling file systems. When this happens, the file system checker must be used to repair the file system. The following provides best practices and other useful information when performing this procedure.

Important

Red Hat does not recommend this unless the machine does not boot, the file system is extremely large, or the file system is on remote storage. It is possible to disable file system check at boot by setting the sixth field in /etc/fstab to 0.

12.1. Best Practices for fsck

Generally, running the file system check and repair tool can be expected to automatically repair at least some of the inconsistencies it finds. In some cases, severely damaged inodes or directories may be discarded if they cannot be repaired. Significant changes to the file system may occur. To ensure that unexpected or undesirable changes are not permanently made, perform the following precautionary steps:
Dry run
Most file system checkers have a mode of operation which checks but does not repair the file system. In this mode, the checker prints any errors that it finds and actions that it would have taken, without actually modifying the file system.

Note

Later phases of consistency checking may print extra errors as it discovers inconsistencies which would have been fixed in early phases if it were running in repair mode.
Operate first on a file system image
Most file systems support the creation of a metadata image, a sparse copy of the file system which contains only metadata. Because file system checkers operate only on metadata, such an image can be used to perform a dry run of an actual file system repair, to evaluate what changes would actually be made. If the changes are acceptable, the repair can then be performed on the file system itself.

Note

Severely damaged file systems may cause problems with metadata image creation.
Save a file system image for support investigations
A pre-repair file system metadata image can often be useful for support investigations if there is a possibility that the corruption was due to a software bug. Patterns of corruption present in the pre-repair image may aid in root-cause analysis.
Operate only on unmounted file systems
A file system repair must be run only on unmounted file systems. The tool must have sole access to the file system or further damage may result. Most file system tools enforce this requirement in repair mode, although some only support check-only mode on a mounted file system. If check-only mode is run on a mounted file system, it may find spurious errors that would not be found when run on an unmounted file system.
Disk errors
File system check tools cannot repair hardware problems. A file system must be fully readable and writable if repair is to operate successfully. If a file system was corrupted due to a hardware error, the file system must first be moved to a good disk, for example with the dd(8) utility.

12.2. File System-Specific Information for fsck

12.2.1. ext2, ext3, and ext4

All of these file sytems use the e2fsck binary to perform file system checks and repairs. The file names fsck.ext2, fsck.ext3, and fsck.ext4 are hardlinks to this same binary. These binaries are run automatically at boot time and their behavior differs based on the file system being checked and the state of the file system.
A full file system check and repair is invoked for ext2, which is not a metadata journaling file system, and for ext4 file systems without a journal.
For ext3 and ext4 file systems with metadata journaling, the journal is replayed in userspace and the binary exits. This is the default action as journal replay ensures a consistent file system after a crash.
If these file systems encounter metadata inconsistencies while mounted, they record this fact in the file system superblock. If e2fsck finds that a file system is marked with such an error, e2fsck performs a full check after replaying the journal (if present).
e2fsck may ask for user input during the run if the -p option is not specified. The -p option tells e2fsck to automatically do all repairs that may be done safely. If user intervention is required, e2fsck indicates the unfixed problem in its output and reflect this status in the exit code.
Commonly used e2fsck run-time options include:
-n
No-modify mode. Check-only operation.
-b superblock
Specify block number of an alternate suprerblock if the primary one is damaged.
-f
Force full check even if the superblock has no recorded errors.
-j journal-dev
Specify the external journal device, if any.
-p
Automatically repair or "preen" the file system with no user input.
-y
Assume an answer of "yes" to all questions.
All options for e2fsck are specified in the e2fsck(8) manual page.
The following five basic phases are performed by e2fsck while running:
  1. Inode, block, and size checks.
  2. Directory structure checks.
  3. Directory connectivity checks.
  4. Reference count checks.
  5. Group summary info checks.
The e2image(8) utility can be used to create a metadata image prior to repair for diagnostic or testing purposes. The -r option should be used for testing purposes in order to create a sparse file of the same size as the file system itself. e2fsck can then operate directly on the resulting file. The -Q option should be specified if the image is to be archived or provided for diagnostic. This creates a more compact file format suitable for transfer.

12.2.2. XFS

No repair is performed automatically at boot time. To initiate a file system check or repair, use the xfs_repair tool.

Note

Although an fsck.xfs binary is present in the xfsprogs package, this is present only to satisfy initscripts that look for an fsck.file system binary at boot time. fsck.xfs immediately exits with an exit code of 0.
Older xfsprogs packages contain an xfs_check tool. This tool is very slow and does not scale well for large file systems. As such, it has been deprecated in favor of xfs_repair -n.
A clean log on a file system is required for xfs_repair to operate. If the file system was not cleanly unmounted, it should be mounted and unmounted prior to using xfs_repair. If the log is corrupt and cannot be replayed, the -L option may be used to zero the log.

Important

The -L option must only be used if the log cannot be replayed. The option discards all metadata updates in the log and results in further inconsistencies.
It is possible to run xfs_repair in a dry run, check-only mode by using the -n option. No changes will be made to the file system when this option is specified.
xfs_repair takes very few options. Commonly used options include:
-n
No modify mode. Check-only operation.
-L
Zero metadata log. Use only if log cannot be replayed with mount.
-m maxmem
Limit memory used during run to maxmem MB. 0 can be specified to obtain a rough estimate of the minimum memory required.
-l logdev
Specify the external log device, if present.
All options for xfs_repair are specified in the xfs_repair(8) manual page.
The following eight basic phases are performed by xfs_repair while running:
  1. Inode and inode blockmap (addressing) checks.
  2. Inode allocation map checks.
  3. Inode size checks.
  4. Directory checks.
  5. Pathname checks.
  6. Link count checks.
  7. Freemap checks.
  8. Super block checks.
For more information, see the xfs_repair(8) manual page.
xfs_repair is not interactive. All operations are performed automatically with no input from the user.
If it is desired to create a metadata image prior to repair for diagnostic or testing purposes, the xfs_metadump(8) and xfs_mdrestore(8) utilities may be used.

12.2.3. Btrfs

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.
The btrfsck tool is used to check and repair btrfs file systems. This tool is still in early development and may not detect or repair all types of file system corruption.
By default, btrfsck does not make changes to the file system; that is, it runs check-only mode by default. If repairs are desired the --repair option must be specified.
The following three basic phases are performed by btrfsck while running:
  1. Extent checks.
  2. File system root checks.
  3. Root reference count checks.
The btrfs-image(8) utility can be used to create a metadata image prior to repair for diagnostic or testing purposes.

Chapter 13. Partitions

Note

For an overview of the advantages and disadvantages to using partitions on block devices, see the following KBase article: https://access.redhat.com/solutions/163853.
With the parted utility, you can:
  • View the existing partition table.
  • Change the size of existing partitions.
  • Add partitions from free space or additional hard drives.
The parted package is installed by default on Red Hat Enterprise Linux 7. To start parted, log in as root and enter the following command:
# parted /dev/sda
Replace /dev/sda with the device name for the drive to configure.

Manipulating Partitions on Devices in Use

For a device to not be in use, none of the partitions on the device can be mounted, and no swap space on the device can be enabled.
If you want to remove or resize a partition, the device on which that partition resides must not be in use.
It is possible to create a new partition on a device that is in use, but this is not recommended.

Modifying the Partition Table

Modifying the partition table while another partition on the same disk is in use is generally not recommended because the kernel is not able to reread the partition table. As a consequence, changes are not applied to a running system. In the described situation, reboot the system, or use the following command to make the system register new or modified partitions:
# partx --update --nr partition-number disk
The easiest way to modify disks that are currently in use is:
  1. Boot the system in rescue mode if the partitions on the disk are impossible to unmount, for example in the case of a system disk.
  2. When prompted to mount the file system, select Skip.
If the drive does not contain any partitions in use, that is there are no system processes that use or lock the file system from being unmounted, you can unmount the partitions with the umount command and turn off all the swap space on the hard drive with the swapoff command.
To see commonly used parted commands, see Table 13.1, “parted Commands”.

Important

Do not use the parted utility to create file systems. Use the mkfs tool instead.
Table 13.1. parted Commands
Command Description
help Display list of available commands
mklabel label Create a disk label for the partition table
mkpart part-type [fs-type] start-mb end-mb Make a partition without creating a new file system
name minor-num name Name the partition for Mac and PC98 disklabels only
print Display the partition table
quit Quit parted
rescue start-mb end-mb Rescue a lost partition from start-mb to end-mb
rm minor-num Remove the partition
select device Select a different device to configure
set minor-num flag state Set the flag on a partition; state is either on or off
toggle [NUMBER [FLAG] Toggle the state of FLAG on partition NUMBER
unit UNIT Set the default unit to UNIT

13.1. Viewing the Partition Table

To view the partition table:
  1. Start parted.
  2. Use the following command to view the partition table:
    (parted) print
A table similar to the following one appears:

Example 13.1. Partition Table

Model: ATA ST3160812AS (scsi)
Disk /dev/sda: 160GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start   End    Size    Type      File system  Flags
 1      32.3kB  107MB  107MB   primary   ext3         boot
 2      107MB   105GB  105GB   primary   ext3
 3      105GB   107GB  2147MB  primary   linux-swap
 4      107GB   160GB  52.9GB  extended		      root
 5      107GB   133GB  26.2GB  logical   ext3
 6      133GB   133GB  107MB   logical   ext3
 7      133GB   160GB  26.6GB  logical                lvm
Following is the description of the partition table:
  • Model: ATA ST3160812AS (scsi): explains the disk type, manufacturer, model number, and interface.
  • Disk /dev/sda: 160GB: displays the file path to the block device and the storage capacity.
  • Partition Table: msdos: displays the disk label type.
  • In the partition table, Number is the partition number. For example, the partition with minor number 1 corresponds to /dev/sda1. The Start and End values are in megabytes. Valid Types are metadata, free, primary, extended, or logical. The File system is the file system type. The Flags column lists the flags set for the partition. Available flags are boot, root, swap, hidden, raid, lvm, or lba.
The File system in the partition table can be any of the following:
  • ext2
  • ext3
  • fat16
  • fat32
  • hfs
  • jfs
  • linux-swap
  • ntfs
  • reiserfs
  • hp-ufs
  • sun-ufs
  • xfs
If a File system of a device shows no value, this means that its file system type is unknown.

Note

To select a different device without having to restart parted, use the following command and replace /dev/sda with the device you want to select:
(parted) select /dev/sda
It allows you to view or configure the partition table of a device.

13.2. Creating a Partition

Warning

Do not attempt to create a partition on a device that is in use.

Procedure 13.1. Creating a Partition

  1. Before creating a partition, boot into rescue mode, or unmount any partitions on the device and turn off any swap space on the device.
  2. Start parted:
    # parted /dev/sda
    Replace /dev/sda with the device name on which you want to create the partition.
  3. View the current partition table to determine if there is enough free space:
    (parted) print
    If there is not enough free space, you can resize an existing partition. For more information, see Section 13.5, “Resizing a Partition with fdisk”.
    From the partition table, determine the start and end points of the new partition and what partition type it should be. You can only have four primary partitions, with no extended partition, on a device. If you need more than four partitions, you can have three primary partitions, one extended partition, and multiple logical partitions within the extended. For an overview of disk partitions, see the appendix An Introduction to Disk Partitions in the Red Hat Enterprise Linux 7 Installation Guide.
  4. To create partition:
    (parted) mkpart part-type name fs-type start end
    Replace part-type with with primary, logical, or extended as per your requirement.
    Replace name with partition-name; name is required for GPT partition tables.
    Replace fs-type with any one of btrfs, ext2, ext3, ext4, fat16, fat32, hfs, hfs+, linux-swap, ntfs, reiserfs, or xfs; fs-type is optional.
    Replace start end with the size in megabytes as per your requirement.
    For example, to create a primary partition with an ext3 file system from 1024 megabytes until 2048 megabytes on a hard drive, type the following command:
    (parted) mkpart primary 1024 2048

    Note

    If you use the mkpartfs command instead, the file system is created after the partition is created. However, parted does not support creating an ext3 file system. Thus, if you wish to create an ext3 file system, use mkpart and create the file system with the mkfs command as described later.
    The changes start taking place as soon as you press Enter, so review the command before executing to it.
  5. View the partition table to confirm that the created partition is in the partition table with the correct partition type, file system type, and size using the following command:
    (parted) print
    Also remember the minor number of the new partition so that you can label any file systems on it.
  6. Exit the parted shell:
    (parted) quit
  7. Use the following command after parted is closed to make sure the kernel recognizes the new partition:
    # cat /proc/partitions 
The maximum number of partitions parted can create is 128. While the GUID Partition Table (GPT) specification allows for more partitions by growing the area reserved for the partition table, common practice used by parted is to limit it to enough area for 128 partitions.

13.2.1. Formatting and Labeling the Partition

To format and label the partition use the following procedure:

Procedure 13.2. Format and Label the Partition

  1. The partition does not have a file system. To create the ext4 file system, use:
    # mkfs.ext4 /dev/sda6

    Warning

    Formatting the partition permanently destroys any data that currently exists on the partition.
  2. Label the file system on the partition. For example, if the file system on the new partition is /dev/sda6 and you want to label it Work, use:
    # e2label /dev/sda6 "Work"
    By default, the installation program uses the mount point of the partition as the label to make sure the label is unique. You can use any label you want.
  3. Create a mount point (e.g. /work) as root.

13.2.2. Add the Partition to /etc/fstab

  1. As root, edit the /etc/fstab file to include the new partition using the partition's UUID.
    Use the command blkid -o list for a complete list of the partition's UUID, or blkid device for individual device details.
    In /etc/fstab:
    • The first column should contain UUID= followed by the file system's UUID.
    • The second column should contain the mount point for the new partition.
    • The third column should be the file system type: for example, ext4 or swap.
    • The fourth column lists mount options for the file system. The word defaults here means that the partition is mounted at boot time with default options.
    • The fifth and sixth field specify backup and check options. Example values for a non-root partition are 0 2.
  2. Regenerate mount units so that your system registers the new configuration:
    # systemctl daemon-reload
  3. Try mounting the file system to verify that the configuration works:
    # mount /work
Additional Information
  • If you need more information about the format of /etc/fstab, see the fstab(5) man page.

13.3. Removing a Partition

Warning

Do not attempt to remove a partition on a device that is in use.

Procedure 13.3. Remove a Partition

  1. Before removing a partition, do one of the following:
    • Boot into rescue mode, or
    • Unmount any partitions on the device and turn off any swap space on the device.
  2. Start the parted utility:
    # parted device
    Replace device with the device on which to remove the partition: for example, /dev/sda.
  3. View the current partition table to determine the minor number of the partition to remove:
    (parted) print
  4. Remove the partition with the command rm. For example, to remove the partition with minor number 3:
    (parted) rm 3
    The changes start taking place as soon as you press Enter, so review the command before committing to it.
  5. After removing the partition, use the print command to confirm that it is removed from the partition table:
    (parted) print
  6. Exit from the parted shell:
    (parted) quit
  7. Examine the content of the /proc/partitions file to make sure the kernel knows the partition is removed:
    # cat /proc/partitions
  8. Remove the partition from the /etc/fstab file. Find the line that declares the removed partition, and remove it from the file.
  9. Regenerate mount units so that your system registers the new /etc/fstab configuration:
    # systemctl daemon-reload

13.4. Setting a Partition Type

The partition type, not to be confused with the file system type, is used by a running system only rarely. However, the partition type matters to on-the-fly generators, such as systemd-gpt-auto-generator, which use the partition type to, for example, automatically identify and mount devices.
You can start the fdisk utility and use the t command to set the partition type. The following example shows how to change the partition type of the first partition to 0x83, default on Linux:
# fdisk /dev/sdc
Command (m for help): t
Selected partition 1
Partition type (type L to list all types): 83
Changed type of partition 'Linux LVM' to 'Linux'.
The parted utility provides some control of partition types by trying to map the partition type to 'flags', which is not convenient for end users. The parted utility can handle only certain partition types, for example LVM or RAID. To remove, for example, the lvm flag from the first partition with parted, use:
# parted /dev/sdc 'set 1 lvm off'
For a list of commonly used partition types and hexadecimal numbers used to represent them, see the Partition Types table in the Partitions: Turning One Drive Into Many appendix of the Red Hat Enterprise Linux 7 Installation Guide.

13.5. Resizing a Partition with fdisk

The fdisk utility allows you to create and manipulate GPT, MBR, Sun, SGI, and BSD partition tables. On disks with a GUID Partition Table (GPT), using the parted utility is recommended, as fdisk GPT support is in an experimental phase.
Before resizing a partition, back up the data stored on the file system and test the procedure, as the only way to change a partition size using fdisk is by deleting and recreating the partition.

Important

The partition you are resizing must be the last partition on a particular disk.
Red Hat only supports extending and resizing LVM partitions.

Procedure 13.4. Resizing a Partition

The following procedure is provided only for reference. To resize a partition using fdisk:
  1. Unmount the device:
    # umount /dev/vda
  2. Run fdisk disk_name. For example:
    # fdisk /dev/vda
    Welcome to fdisk (util-linux 2.23.2).
    
    Changes will remain in memory only, until you decide to write them. Be careful before using the write command.
    
    Command (m for help):
    
  3. Use the p option to determine the line number of the partition to be deleted.
    Command (m for help): p
    Disk /dev/vda: 16.1 GB, 16106127360 bytes, 31457280 sectors
    Units = sectors of 1 * 512 = 512 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disk label type: dos
    Disk identifier: 0x0006d09a
    
    Device    Boot      Start         End      Blocks   Id  System
    /dev/vda1   *        2048     1026047      512000   83  Linux
    /dev/vda2         1026048    31457279    15215616   8e  Linux LVM
    
  4. Use the d option to delete a partition. If there is more than one partition available, fdisk prompts you to provide a number of the partition to delete:
    Command (m for help): d
    Partition number (1,2, default 2): 2
    Partition 2 is deleted
    
  5. Use the n option to create a partition and follow the prompts. Allow enough space for any future resizing. The fdisk default behavior (press Enter) is to use all space on the device. You can specify the end of the partition by sectors, or specify a human-readable size by using +<size><suffix>, for example +500M, or +10G.
    Red Hat recommends using the human-readable size specification if you do not want to use all free space, as fdisk aligns the end of the partition with the physical sectors. If you specify the size by providing an exact number (in sectors), fdisk does not align the end of the partition.
    Command (m for help): n
    Partition type:
       p   primary (1 primary, 0 extended, 3 free)
       e   extended
    Select (default p): *Enter*
    Using default response p
    Partition number (2-4, default 2): *Enter*
    First sector (1026048-31457279, default 1026048): *Enter*
    Using default value 1026048
    Last sector, +sectors or +size{K,M,G} (1026048-31457279, default 31457279): +500M
    Partition 2 of type Linux and of size 500 MiB is set
    
  6. Set the partition type to LVM:
    Command (m for help): t
    Partition number (1,2, default 2): *Enter*
    Hex code (type L to list all codes): 8e
    Changed type of partition 'Linux' to 'Linux LVM'
    
  7. Write the changes with the w option when you are sure the changes are correct, as errors can cause instability with the selected partition.
  8. Run e2fsck on the device to check for consistency:
    # e2fsck /dev/vda
    e2fsck 1.41.12 (17-May-2010)
    Pass 1:Checking inodes, blocks, and sizes
    Pass 2:Checking directory structure
    Pass 3:Checking directory connectivity
    Pass 4:Checking reference counts
    Pass 5:Checking group summary information
    ext4-1:11/131072 files (0.0% non-contiguous),27050/524128 blocks
    
  9. Mount the device:
    # mount /dev/vda
For more information, see the fdisk(8) manual page.

Chapter 14. Creating and Maintaining Snapshots with Snapper

A snapshot volume is a point in time copy of a target volume that provides a way to revert a file system back to an earlier state. Snapper is a command-line tool to create and maintain snapshots for Btrfs and thinly-provisioned LVM file systems.

14.1. Creating Initial Snapper Configuration

Snapper requires discrete configuration files for each volume it operates on. You must set up the configuration files manually. By default, only the root user is allowed to perform snapper commands.

Warning

If you are using thin provisioning, monitor free space in the thin-pool. A fully consumed thin pool can result in data loss. For more information, see Thinly-Provisioned Logical Volumes (Thin Volumes) in the Red Hat Enterprise Linux 7 Logical Volume Manager Administration Guide.
Note that the Btrfs tools and file system are provided as a Technology Preview, which make them unsuitable for production systems.
Although it is possible to allow a user or group other than root to use certain Snapper commands, Red Hat recommends that you do not add elevated permissions to otherwise unprivileged users or groups. Such a configuration bypasses SELinux and could pose a security risk. Red Hat recommends that you review these capabilities with your Security Team and consider using the sudo infrastructure instead.

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.

Procedure 14.1. Creating a Snapper Configuration File

  1. Create or choose either:
    • A thinly-provisioned logical volume with a Red Hat supported file system on top of it, or
    • A Btrfs subvolume.
  2. Mount the file system.
  3. Create the configuration file that defines this volume.
    For LVM2:
    # snapper -c config_name create-config -f "lvm(fs_type)" /mount-point
    For example, to create a configuration file called lvm_config on an LVM2 subvolume with an ext4 file system, mounted at /lvm_mount, use:
    # snapper -c lvm_config create-config -f "lvm(ext4)" /lvm_mount
    For Btrfs:
    # snapper -c config_name create-config -f btrfs /mount-point
    • The -c config_name option specifies the name of the configuration file.
    • The create-config tells snapper to create a configuration file.
    • The -f file_system tells snapper what file system to use; if this is omitted snapper will attempt to detect the file system.
    • The /mount-point is where the subvolume or thinly-provisioned LVM2 file system is mounted.
    Alternatively, to create a configuration file called btrfs_config, on a Btrfs subvolume that is mounted at /btrfs_mount, use:
    # snapper -c btrfs_config create-config -f btrfs /btrfs_mount
The configuration files are stored in the /etc/snapper/configs/ directory.

14.2. Creating a Snapper Snapshot

Snapper can create the following kinds of snapshots:
Pre Snapshot
A pre snapshot serves as a point of origin for a post snapshot. The two are closely tied and designed to track file system modification between the two points. The pre snapshot must be created before the post snapshot.
Post Snapshot
A post snapshot serves as the end point to the pre snapshot. The coupled pre and post snapshots define a range for comparison. By default, every new snapper volume is configured to create a background comparison after a related post snapshot is created successfully.
Single Snapshot
A single snapshot is a standalone snapshot created at a specific moment. These can be used to track a timeline of modifications and have a general point to return to later.

14.2.1. Creating a Pre and Post Snapshot Pair

14.2.1.1. Creating a Pre Snapshot with Snapper
To create a pre snapshot, use:
# snapper -c config_name create -t pre
The -c config_name option creates a snapshot according to the specifications in the named configuration file. If the configuration file does not yet exist, see Section 14.1, “Creating Initial Snapper Configuration”.
The create -t option specifies what type of snapshot to create. Accepted entries are pre, post, or single.
For example, to create a pre snapshot using the lvm_config configuration file, as created in Section 14.1, “Creating Initial Snapper Configuration”, use:
# snapper -c SnapperExample create -t pre -p
1
The -p option prints the number of the created snapshot and is optional.
14.2.1.2. Creating a Post Snapshot with Snapper
A post snapshot is the end point of the snapshot and should be created after the parent pre snapshot by following the instructions in Section 14.2.1.1, “Creating a Pre Snapshot with Snapper”.

Procedure 14.2. Creating a Post Snapshot

  1. Determine the number of the pre snapshot:
    # snapper -c config_name list
    For example, to display the list of snapshots created using the configuration file lvm_config, use the following:
    # snapper -c lvm_config list
    Type   | # | Pre # | Date              | User | Cleanup  | Description | Userdata
    -------+---+-------+-------------------+------+----------+-------------+---------
    single | 0 |       |                   | root |          | current     |
    pre    | 1 |       | Mon 06<...>       | root |          |             |
    
    This output shows that the pre snapshot is number 1.
  2. Create a post snapshot that is linked to a previously created pre snapshot:
    # snapper -c config_file create -t post --pre-num pre_snapshot_number
    • The -t post option specifies the creation of the post snapshot type.
    • The --pre-num option specifies the corresponding pre snapshot.
    For example, to create a post snapshot using the lvm_config configuration file and is linked to pre snapshot number 1, use:
    # snapper -c lvm_config create -t post --pre-num 1 -p
    2
    
    The -p option prints the number of the created snapshot and is optional.
  3. The pre and post snapshots 1 and 2 are now created and paired. Verify this with the list command:
    # snapper -c lvm_config list
    Type   | # | Pre # | Date              | User | Cleanup  | Description | Userdata
    -------+---+-------+-------------------+------+----------+-------------+---------
    single | 0 |       |                   | root |          | current     |
    pre    | 1 |       | Mon 06<...>       | root |          |             |
    post   | 2 | 1     | Mon 06<...>       | root |          |             |
    
14.2.1.3. Wrapping a Command in Pre and Post Snapshots
You can also wrap a command within a pre and post snapshot, which can be useful when testing. See Procedure 14.3, “Wrapping a Command in Pre and Post Snapshots”, which is a shortcut for the following steps:
  1. Running the snapper create pre snapshot command.
  2. Running a command or a list of commands to perform actions with a possible impact on the file system content.
  3. Running the snapper create post snapshot command.

Procedure 14.3. Wrapping a Command in Pre and Post Snapshots

  1. To wrap a command in pre and post snapshots:
    # snapper -c lvm_config create --command "command_to_be_tracked"
    For example, to track the creation of the /lvm_mount/hello_file file:
    # snapper -c lvm_config create --command "echo Hello > /lvm_mount/hello_file"
  2. To verify this, use the status command:
    # snapper -c config_file status first_snapshot_number..second_snapshot_number
    For example, to track the changes made in the first step:
    # snapper -c lvm_config status 3..4
    +..... /lvm_mount/hello_file
    
    Use the list command to verify the number of the snapshot if needed.
    For more information on the status command, see Section 14.3, “Tracking Changes Between Snapper Snapshots”.
Note that there is no guarantee that the command in the given example is the only thing the snapshots capture. Snapper also records anything that is modified by the system, not just what a user modifies.

14.2.2. Creating a Single Snapper Snapshot

Creating a single snapper snapshot is similar to creating a pre or post snapshot, only the create -t option specifies single. The single snapshot is used to create a single snapshot in time without having it relate to any others. However, if you are interested in a straightforward way to create snapshots of LVM2 thin volumes without the need to automatically generate comparisons or list additional information, Red Hat recommends using the System Storage Manager instead of Snapper for this purpose, as described in Section 16.2.6, “Snapshot”.
To create a single snapshot, use:
# snapper -c config_name create -t single
For example, the following command creates a single snapshot using the lvm_config configuration file.
# snapper -c lvm_config create -t single
Although single snapshots are not specifically designed to track changes, you can use the snapper diff, xadiff, and status commands to compare any two snapshots. For more information on these commands, see Section 14.3, “Tracking Changes Between Snapper Snapshots”.

14.2.3. Configuring Snapper to Take Automated Snapshots

Taking automated snapshots is one of key features of Snapper. By default, when you configure Snapper for a volume, Snapper starts taking a snapshot of the volume every hour.
Under the default configuration, Snapper keeps:
  • 10 hourly snapshots, and the final hourly snapshot is saved as a “daily” snapshot.
  • 10 daily snapshots, and the final daily snapshot for a month is saved as a “monthly” snapshot.
  • 10 monthly snapshots, and the final monthly snapshot is saved as a “yearly” snapshot.
  • 10 yearly snapshots.
Note that Snapper keeps by default no more that 50 snapshots in total. However, Snapper keeps by default all snapshots created less than 1,800 seconds ago.
The default configuration is specified in the /etc/snapper/config-templates/default file. When you use the snapper create-config command to create a configuration, any unspecified values are set based on the default configuration. You can edit the configuration for any defined volume in the /etc/snapper/configs/config_name file.

14.3. Tracking Changes Between Snapper Snapshots

Use the status, diff, and xadiff commands to track the changes made to a subvolume between snapshots:
status
The status command shows a list of files and directories that have been created, modified, or deleted between two snapshots, that is a comprehensive list of changes between two snapshots. You can use this command to get an overview of the changes without excessive details.
diff
The diff command shows a diff of modified files and directories between two snapshots as received from the status command if there is at least one modification detected.
xadiff
The xadiff command compares how the extended attributes of a file or directory have changed between two snapshots.

14.3.1. Comparing Changes with the status Command

The status command shows a list of files and directories that have been created, modified, or deleted between two snapshots.
To display the status of files between two snapshots, use:
# snapper -c config_file status first_snapshot_number..second_snapshot_number
Use the list command to determine snapshot numbers if needed.
For example, the following command displays the changes made between snapshot 1 and 2, using the configuration file lvm_config.
#snapper -c lvm_config status 1..2
tp.... /lvm_mount/dir1
-..... /lvm_mount/dir1/file_a
c.ug.. /lvm_mount/file2
+..... /lvm_mount/file3
....x. /lvm_mount/file4
cp..xa /lvm_mount/file5
Read letters and dots in the first part of the output as columns:
+..... /lvm_mount/file3
||||||
123456
Column 1 indicates any modification of the file (directory entry) type. Possible values are:
Column 1
OutputMeaning
.Nothing has changed.
+File created.
-File deleted.
cContent changed.
tThe type of directory entry has changed. For example, a former symbolic link has changed to a regular file with the same file name.
Column 2 indicates any changes in the file permissions. Possible values are:
Column 2
OutputMeaning
.No permissions changed.
pPermissions changed.
Column 3 indicates any changes in the user ownership. Possible values are:
Column 3
OutputMeaning
.No user ownership changed.
uUser ownership has changed.
Column 4 indicates any changes in the group ownership. Possible values are:
Column 4
OutputMeaning
.No group ownership changed.
gGroup ownership has changed.
Column 5 indicates any changes in the extended attributes. Possible values are:
Column 5
OutputMeaning
.No extended attributes changed.
xExtended attributes changed.
Column 6 indicates any changes in the access control lists (ACLs). Possible values are:
Column 6
OutputMeaning
.No ACLs changed.
aACLs modified.

14.3.2. Comparing Changes with the diff Command

The diff command shows the changes of modified files and directories between two snapshots.
# snapper -c config_name diff first_snapshot_number..second_snapshot_number
Use the list command to determine the number of the snapshot if needed.
For example, to compare the changes made in files between snapshot 1 and snapshot 2 that were made using the lvm_config configuration file, use:
# snapper -c lvm_config diff 1..2
--- /lvm_mount/.snapshots/13/snapshot/file4	19<...>
+++ /lvm_mount/.snapshots/14/snapshot/file4	20<...>
@@ -0,0 +1 @@
+words
This output shows that file4 had been modified to add "words" into the file.

14.3.3. Comparing Changes with the xadiff Command

The xadiff command compares how the extended attributes of a file or directory have changed between two snapshots:
# snapper -c config_name xadiff first_snapshot_number..second_snapshot_number
Use the list command to determine the number of the snapshot if needed.
For example, to show the xadiff output between snapshot number 1 and snapshot number 2 that were made using the lvm_config configuration file, use:
# snapper -c lvm_config xadiff 1..2

14.4. Reversing Changes in Between Snapshots

To reverse changes made between two existing Snapper snapshots, use the undochange command in the following format, where 1 is the first snapshot and 2 is the second snapshot:
snapper -c config_name undochange 1..2

Important

Using the undochange command does not revert the Snapper volume back to its original state and does not provide data consistency. Any file modification that occurs outside of the specified range, for example after snapshot 2, will remain unchanged after reverting back, for example to the state of snapshot 1. For example, if undochange is run to undo the creation of a user, any files owned by that user can still remain.
There is also no mechanism to ensure file consistency as a snapshot is made, so any inconsistencies that already exist can be transferred back to the snapshot when the undochange command is used.
Do not use the Snapper undochange command with the root file system, as doing so is likely to lead to a failure.
The following diagram demonstrates how the undochange command works:
Snapper Status over Time

Figure 14.1. Snapper Status over Time

The diagram shows the point in time in which snapshot_1 is created, file_a is created, then file_b deleted. Snapshot_2 is then created, after which file_a is edited and file_c is created. This is now the current state of the system. The current system has an edited version of file_a, no file_b, and a newly created file_c.
When the undochange command is called, Snapper generates a list of modified files between the first listed snapshot and the second. In the diagram, if you use the snapper -c SnapperExample undochange 1..2 command, Snapper creates a list of modified files (that is, file_a is created; file_b is deleted) and applies them to the current system. Therefore:
  • the current system will not have file_a, as it has yet to be created when snapshot_1 was created.
  • file_b will exist, copied from snapshot_1 into the current system.
  • file_c will exist, as its creation was outside the specified time.
Be aware that if file_b and file_c conflict, the system can become corrupted.
You can also use the snapper -c SnapperExample undochange 2..1 command. In this case, the current system replaces the edited version of file_a with one copied from snapshot_1, which undoes edits of that file made after snapshot_2 was created.

Using the mount and unmount Commands to Reverse Changes

The undochange command is not always the best way to revert modifications. With the status and diff command, you can make a qualified decision, and use the mount and unmount commands instead of Snapper. The mount and unmount commands are only useful if you want to mount snapshots and browse their content independently of Snapper workflow.
If needed, the mount command activates respective LVM Snapper snapshot before mounting. Use the mount and unmount commands if you are, for example, interested in mounting snapshots and extracting older version of several files manually. To revert files manually, copy them from a mounted snapshot to the current file system. The current file system, snapshot 0, is the live file system created in Procedure 14.1, “Creating a Snapper Configuration File”. Copy the files to the subtree of the original /mount-point.
Use the mount and unmount commands for explicit client-side requests. The /etc/snapper/configs/config_name file contains the ALLOW_USERS= and ALLOW_GROUPS= variables where you can add users and groups. Then, snapperd allows you to perform mount operations for the added users and groups.

14.5. Deleting a Snapper Snapshot

To delete a snapshot:
# snapper -c config_name delete snapshot_number
You can use the list command to verify that the snapshot was successfully deleted.

Chapter 15. Swap Space

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM. Swap space is located on hard drives, which have a slower access time than physical memory. Swap space can be a dedicated swap partition (recommended), a swap file, or a combination of swap partitions and swap files. Note that Btrfs does not support swap space.
In years past, the recommended amount of swap space increased linearly with the amount of RAM in the system. However, modern systems often include hundreds of gigabytes of RAM. As a consequence, recommended swap space is considered a function of system memory workload, not system memory.
Table 15.1, “Recommended System Swap Space” illustrates the recommended size of a swap partition depending on the amount of RAM in your system and whether you want sufficient memory for your system to hibernate. The recommended swap partition size is established automatically during installation. To allow for hibernation, however, you need to edit the swap space in the custom partitioning stage.
Recommendations in Table 15.1, “Recommended System Swap Space” are especially important on systems with low memory (1 GB and less). Failure to allocate sufficient swap space on these systems can cause issues such as instability or even render the installed system unbootable.
Table 15.1. Recommended System Swap Space
Amount of RAM in the system Recommended swap space Recommended swap space if allowing for hibernation
⩽ 2 GB 2 times the amount of RAM 3 times the amount of RAM
> 2 GB – 8 GB Equal to the amount of RAM 2 times the amount of RAM
> 8 GB – 64 GB At least 4 GB 1.5 times the amount of RAM
> 64 GB At least 4 GB Hibernation not recommended

Note

There are two reasons why hibernation is not recommended with systems with more than 64 GB of RAM. Firstly, hibernation requires extra space for an inflated (and perhaps infrequently utilized) swap area. Secondly, the process of moving resident data from RAM to disk and back on can take a lot of time to complete.
At the border between each range listed in Table 15.1, “Recommended System Swap Space”, for example a system with 2 GB, 8 GB, or 64 GB of system RAM, discretion can be exercised with regard to chosen swap space and hibernation support. If your system resources allow for it, increasing the swap space may lead to better performance.
Note that distributing swap space over multiple storage devices also improves swap space performance, particularly on systems with fast drives, controllers, and interfaces.

Important

File systems and LVM2 volumes assigned as swap space should not be in use when being modified. Any attempts to modify swap fail if a system process or the kernel is using swap space. Use the free and cat /proc/swaps commands to verify how much and where swap is in use.
You should modify swap space while the system is booted in rescue mode, see Booting Your Computer in Rescue Mode in the Red Hat Enterprise Linux 7 Installation Guide. When prompted to mount the file system, select Skip.

15.1. Adding Swap Space

Sometimes it is necessary to add more swap space after installation. For example, you may upgrade the amount of RAM in your system from 1 GB to 2 GB, but there is only 2 GB of swap space. It might be advantageous to increase the amount of swap space to 4 GB if you perform memory-intense operations or run applications that require a large amount of memory.
You have three options: create a new swap partition, create a new swap file, or extend swap on an existing LVM2 logical volume. It is recommended that you extend an existing logical volume.

15.1.1. Extending Swap on an LVM2 Logical Volume

By default, Red Hat Enterprise Linux 7 uses all available space during installation. If this is the case with your system, then you must first add a new physical volume to the volume group used by the swap space.
After adding additional storage to the swap space's volume group, it is now possible to extend it. To do so, perform the following procedure (assuming /dev/VolGroup00/LogVol01 is the volume you want to extend by 2 GB):

Procedure 15.1. Extending Swap on an LVM2 Logical Volume

  1. Disable swapping for the associated logical volume:
    # swapoff -v /dev/VolGroup00/LogVol01
  2. Resize the LVM2 logical volume by 2 GB:
    # lvresize /dev/VolGroup00/LogVol01 -L +2G
  3. Format the new swap space:
    # mkswap /dev/VolGroup00/LogVol01
  4. Enable the extended logical volume:
    # swapon -v /dev/VolGroup00/LogVol01
  5. To test if the swap logical volume was successfully extended and activated, inspect active swap space:
    $ cat /proc/swaps
    $ free -h

15.1.2. Creating an LVM2 Logical Volume for Swap

To add a swap volume group 2 GB in size, assuming /dev/VolGroup00/LogVol02 is the swap volume you want to add:
  1. Create the LVM2 logical volume of size 2 GB:
    # lvcreate VolGroup00 -n LogVol02 -L 2G
  2. Format the new swap space:
    # mkswap /dev/VolGroup00/LogVol02
  3. Add the following entry to the /etc/fstab file:
    /dev/VolGroup00/LogVol02   swap     swap    defaults     0 0
  4. Regenerate mount units so that your system registers the new configuration:
    # systemctl daemon-reload
  5. Activate swap on the logical volume:
    # swapon -v /dev/VolGroup00/LogVol02
  6. To test if the swap logical volume was successfully created and activated, inspect active swap space:
    $ cat /proc/swaps
    $ free -h

15.1.3. Creating a Swap File

To add a swap file:

Procedure 15.2. Add a Swap File

  1. Determine the size of the new swap file in megabytes and multiply by 1024 to determine the number of blocks. For example, the block size of a 64 MB swap file is 65536.
  2. Create an empty file:
    # dd if=/dev/zero of=/swapfile bs=1024 count=65536
    Replace count with the value equal to the desired block size.
  3. Set up the swap file with the command:
    # mkswap /swapfile
  4. Change the security of the swap file so it is not world readable.
    # chmod 0600 /swapfile
  5. To enable the swap file at boot time, edit /etc/fstab as root to include the following entry:
    /swapfile          swap            swap    defaults        0 0
    The next time the system boots, it activates the new swap file.
  6. Regenerate mount units so that your system registers the new /etc/fstab configuration:
    # systemctl daemon-reload
  7. To activate the swap file immediately:
    # swapon /swapfile
  8. To test if the new swap file was successfully created and activated, inspect active swap space:
    $ cat /proc/swaps
    $ free -h

15.2. Removing Swap Space

Sometimes it can be prudent to reduce swap space after installation. For example, you have downgraded the amount of RAM in your system from 1 GB to 512 MB, but there is 2 GB of swap space still assigned. It might be advantageous to reduce the amount of swap space to 1 GB, since the larger 2 GB could be wasting disk space.
You have three options: remove an entire LVM2 logical volume used for swap, remove a swap file, or reduce swap space on an existing LVM2 logical volume.

15.2.1. Reducing Swap on an LVM2 Logical Volume

To reduce an LVM2 swap logical volume (assuming /dev/VolGroup00/LogVol01 is the volume you want to reduce):

Procedure 15.3. Reducing an LVM2 Swap Logical Volume

  1. Disable swapping for the associated logical volume:
    # swapoff -v /dev/VolGroup00/LogVol01
  2. Reduce the LVM2 logical volume by 512 MB:
    # lvreduce /dev/VolGroup00/LogVol01 -L -512M
  3. Format the new swap space:
    # mkswap /dev/VolGroup00/LogVol01
  4. Activate swap on the logical volume:
    # swapon -v /dev/VolGroup00/LogVol01
  5. To test if the swap logical volume was successfully reduced, inspect active swap space:
    $ cat /proc/swaps
    $ free -h

15.2.2. Removing an LVM2 Logical Volume for Swap

To remove a swap volume group (assuming /dev/VolGroup00/LogVol02 is the swap volume you want to remove):

Procedure 15.4. Remove a Swap Volume Group

  1. Disable swapping for the associated logical volume:
    # swapoff -v /dev/VolGroup00/LogVol02
  2. Remove the LVM2 logical volume:
    # lvremove /dev/VolGroup00/LogVol02
  3. Remove the following associated entry from the /etc/fstab file:
    /dev/VolGroup00/LogVol02   swap     swap    defaults     0 0
  4. Regenerate mount units so that your system registers the new configuration:
    # systemctl daemon-reload
  5. Remove all references to the removed swap storage from the /etc/default/grub file:
    # vi /etc/default/grub
  6. Rebuild the grub configuration:
    1. on BIOS-based machines, run:
      # grub2-mkconfig -o /boot/grub2/grub.cfg
    2. on UEFI-based machines, run:
      # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
  7. To test if the logical volume was successfully removed, inspect active swap space:
    $ cat /proc/swaps
    $ free -h

15.2.3. Removing a Swap File

To remove a swap file:

Procedure 15.5. Remove a Swap File

  1. At a shell prompt, execute the following command to disable the swap file (where /swapfile is the swap file):
    # swapoff -v /swapfile
  2. Remove its entry from the /etc/fstab file.
  3. Regenerate mount units so that your system registers the new configuration:
    # systemctl daemon-reload
  4. Remove the actual file:
    # rm /swapfile

15.3. Moving Swap Space

To move swap space from one location to another:

Chapter 16. System Storage Manager (SSM)

System Storage Manager (SSM) provides a command line interface to manage storage in various technologies. Storage systems are becoming increasingly complicated through the use of Device Mappers (DM), Logical Volume Managers (LVM), and Multiple Devices (MD). This creates a system that is not user friendly and makes it easier for errors and problems to arise. SSM alleviates this by creating a unified user interface. This interface allows users to run complicated systems in a simple manner. For example, to create and mount a new file system without SSM, there are five commands that must be used. With SSM only one is needed.
This chapter explains how SSM interacts with various back ends and some common use cases.

16.1. SSM Back Ends

SSM uses a core abstraction layer in ssmlib/main.py which complies with the device, pool, and volume abstraction, ignoring the specifics of the underlying technology. Back ends can be registered in ssmlib/main.py to handle specific storage technology methods, such as create, snapshot, or to remove volumes and pools.
There are already several back ends registered in SSM. The following sections provide basic information on them as well as definitions on how they handle pools, volumes, snapshots, and devices.

16.1.1. Btrfs Back End

Note

Btrfs is available as a Technology Preview feature in Red Hat Enterprise Linux 7 but has been deprecated since the Red Hat Enterprise Linux 7.4 release. It will be removed in a future major release of Red Hat Enterprise Linux.
For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.4 Release Notes.
Btrfs, a file system with many advanced features, is used as a volume management back end in SSM. Pools, volumes, and snapshots can be created with the Btrfs back end.
16.1.1.1. Btrfs Pool
The Btrfs file system itself is the pool. It can be extended by adding more devices or shrunk by removing devices. SSM creates a Btrfs file system when a Btrfs pool is created. This means that every new Btrfs pool has one volume of the same name as the pool which cannot be removed without removing the entire pool. The default Btrfs pool name is btrfs_pool.
The name of the pool is used as the file system label. If there is already an existing Btrfs file system in the system without a label, the Btrfs pool will generate a name for internal use in the format of btrfs_device_base_name.
16.1.1.2. Btrfs Volume
Volumes created after the first volume in a pool are the same as sub-volumes. SSM temporarily mounts the Btrfs file system if it is unmounted in order to create a sub-volume.
The name of a volume is used as the subvolume path in the Btrfs file system. For example, a subvolume displays as /dev/lvm_pool/lvol001. Every object in this path must exist in order for the volume to be created. Volumes can also be referenced with its mount point.
16.1.1.3. Btrfs Snapshot
Snapshots can be taken of any Btrfs volume in the system with SSM. Be aware that Btrfs does not distinguish between subvolumes and snapshots. While this means that SSM cannot recognize the Btrfs snapshot destination, it will try to recognize special name formats. If the name specified when creating the snapshot does the specific pattern, the snapshot is not be recognized and instead be listed as a regular Btrfs volume.
16.1.1.4. Btrfs Device
Btrfs does not require any special device to be created on.

16.1.2. LVM Back End

Pools, volumes, and snapshots can be created with LVM. The following definitions are from an LVM point of view.
16.1.2.1. LVM Pool
LVM pool is the same as an LVM volume group. This means that grouping devices and new logical volumes can be created out of the LVM pool. The default LVM pool name is lvm_pool.
16.1.2.2. LVM Volume
An LVM volume is the same as an ordinary logical volume.
16.1.2.3. LVM Snapshot
When a snapshot is created from the LVM volume a new snapshot volume is created which can then be handled just like any other LVM volume. Unlike Btrfs, LVM is able to distinguish snapshots from regular volumes so there is no need for a snapshot name to match a particular pattern.
16.1.2.4. LVM Device
SSM makes the need for an LVM back end to be created on a physical device transparent for the user.

16.1.3. Crypt Back End

The crypt back end in SSM uses cryptsetup and dm-crypt target to manage encrypted volumes. Crypt back ends can be used as a regular back end for creating encrypted volumes on top of regular block devices (or on other volumes such as LVM or MD volumes), or to create encrypted LVM volumes in a single steps.
Only volumes can be created with a crypt back end; pooling is not supported and it does not require special devices.
The following sections define volumes and snapshots from the crypt point of view.
16.1.3.1. Crypt Volume
Crypt volumes are created by dm-crypt and represent the data on the original encrypted device in an unencrypted form. It does not support RAID or any device concatenation.
Two modes, or extensions, are supported: luks and plain. Luks is used by default. For more information on the extensions, see man cryptsetup.
16.1.3.2. Crypt Snapshot
While the crypt back end does not support snapshotting, if the encrypted volume is created on top of an LVM volume, the volume itself can be snapshotted. The snapshot can then be opened by using cryptsetup.

16.1.4. Multiple Devices (MD) Back End

MD back end is currently limited to only gathering the information about MD volumes in the system.

16.2. Common SSM Tasks

The following sections describe common SSM tasks.

16.2.1. Installing SSM

To install SSM use the following command:
# yum install system-storage-manager
There are several back ends that are enabled only if the supporting packages are installed:
  • The LVM back end requires the lvm2 package.
  • The Btrfs back end requires the btrfs-progs package.
  • The Crypt back end requires the device-mapper and cryptsetup packages.

16.2.2. Displaying Information about All Detected Devices

Displaying information about all detected devices, pools, volumes, and snapshots is done with the list command. The ssm list command with no options display the following output:
# ssm list
----------------------------------------------------------
Device        Free      Used      Total  Pool  Mount point
----------------------------------------------------------
/dev/sda                        2.00 GB        PARTITIONED
/dev/sda1                      47.83 MB        /test
/dev/vda                       15.00 GB        PARTITIONED
/dev/vda1                     500.00 MB        /boot
/dev/vda2  0.00 KB  14.51 GB   14.51 GB  rhel
----------------------------------------------------------
------------------------------------------------
Pool  Type  Devices     Free      Used     Total
------------------------------------------------
rhel  lvm   1        0.00 KB  14.51 GB  14.51 GB
------------------------------------------------
---------------------------------------------------------------------------------
Volume          Pool  Volume size  FS     FS size       Free  Type    Mount point
---------------------------------------------------------------------------------
/dev/rhel/root  rhel     13.53 GB  xfs   13.52 GB    9.64 GB  linear  /
/dev/rhel/swap  rhel   1000.00 MB                             linear
/dev/sda1                47.83 MB  xfs   44.50 MB   44.41 MB  part    /test
/dev/vda1               500.00 MB  xfs  496.67 MB  403.56 MB  part    /boot
---------------------------------------------------------------------------------
This display can be further narrowed down by using arguments to specify what should be displayed. The list of available options can be found with the ssm list --help command.

Note

Depending on the argument given, SSM may not display everything.
  • Running the devices or dev argument omits some devices. CDRoms and DM/MD devices, for example, are intentionally hidden as they are listed as volumes.
  • Some back ends do not support snapshots and cannot distinguish between a snapshot and a regular volume. Running the snapshot argument on one of these back ends cause SSM to attempt to recognize the volume name in order to identify a snapshot. If the SSM regular expression does not match the snapshot pattern then the snapshot is not be recognized.
  • With the exception of the main Btrfs volume (the file system itself), any unmounted Btrfs volumes are not shown.

16.2.3. Creating a New Pool, Logical Volume, and File System

In this section, a new pool is being created with a default name which have the devices /dev/vdb and /dev/vdc, a logical volume of 1G, and an XFS file system.
The command to create this scenario is ssm create --fs xfs -s 1G /dev/vdb /dev/vdc. The following options are used:
  • The --fs option specifies the required file system type. Current supported file system types are:
    • ext3
    • ext4
    • xfs
    • btrfs
  • The -s specifies the size of the logical volume. The following suffixes are supported to define units:
    • K or k for kilobytes
    • M or m for megabytes
    • G or g for gigabytes
    • T or t for terabytes
    • P or p for petabytes
    • E or e for exabytes
  • Additionaly, with the -s option, the new size can be specified as a percentage. Look at the examples:
    • 10% for 10 percent of the total pool size
    • 10%FREE for 10 percent of the free pool space
    • 10%USED for 10 percent of the used pool space
The two listed devices, /dev/vdb and /dev/vdc, are the two devices you wish to create.
# ssm create --fs xfs -s 1G /dev/vdb /dev/vdc
  Physical volume "/dev/vdb" successfully created
  Physical volume "/dev/vdc" successfully created
  Volume group "lvm_pool" successfully created
  Logical volume "lvol001" created
There are two other options for the ssm command that may be useful. The first is the -p pool command. This specifies the pool the volume is to be created on. If it does not yet exist, then SSM creates it. This was omitted in the given example which caused SSM to use the default name lvm_pool. However, to use a specific name to fit in with any existing naming conventions, the -p option should be used.
The second useful option is the -n name command. This names the newly created logical volume. As with the -p, this is needed in order to use a specific name to fit in with any existing naming conventions.
An example of these two options being used follows:
# ssm create --fs xfs -p new_pool -n XFS_Volume /dev/vdd
  Volume group "new_pool" successfully created
  Logical volume "XFS_Volume" created
SSM has now created two physical volumes, a pool, and a logical volume with the ease of only one command.

16.2.4. Checking a File System's Consistency

The ssm check command checks the file system consistency on the volume. It is possible to specify multiple volumes to check. If there is no file system on the volume, then the volume is skipped.
To check all devices in the volume lvol001, run the command ssm check /dev/lvm_pool/lvol001.
# ssm check /dev/lvm_pool/lvol001
Checking xfs file system on '/dev/mapper/lvm_pool-lvol001'.
Phase 1 - find and verify superblock...
Phase 2 - using internal log
        - scan filesystem freespace and inode maps...
        - found root inode chunk
Phase 3 - for each AG...
        - scan (but don't clear) agi unlinked lists...
        - process known inodes and perform inode discovery...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
        - agno = 4
        - agno = 5
        - agno = 6
        - process newly discovered inodes...
Phase 4 - check for duplicate blocks...
        - setting up duplicate extent list...
        - check for inodes claiming duplicate blocks...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
        - agno = 4
        - agno = 5
        - agno = 6
No modify flag set, skipping phase 5
Phase 6 - check inode connectivity...
        - traversing filesystem ...
        - traversal finished ...
        - moving disconnected inodes to lost+found ...
Phase 7 - verify link counts...
No modify flag set, skipping filesystem flush and exiting.

16.2.5. Increasing a Volume's Size

The ssm resize command changes the size of the specified volume and file system. If there is no file system then only the volume itself will be resized.
For this example, we currently have one logical volume on /dev/vdb that is 900MB called lvol001.
# ssm list
-----------------------------------------------------------------
Device          Free       Used      Total  Pool      Mount point
-----------------------------------------------------------------
/dev/vda                          15.00 GB            PARTITIONED
/dev/vda1                        500.00 MB            /boot
/dev/vda2    0.00 KB   14.51 GB   14.51 GB  rhel
/dev/vdb   120.00 MB  900.00 MB    1.00 GB  lvm_pool
/dev/vdc                           1.00 GB
-----------------------------------------------------------------
---------------------------------------------------------
Pool      Type  Devices       Free       Used       Total
---------------------------------------------------------
lvm_pool  lvm   1        120.00 MB  900.00 MB  1020.00 MB
rhel      lvm   1          0.00 KB   14.51 GB    14.51 GB
---------------------------------------------------------
--------------------------------------------------------------------------------------------
Volume                 Pool      Volume size  FS     FS size       Free  Type    Mount point
--------------------------------------------------------------------------------------------
/dev/rhel/root         rhel         13.53 GB  xfs   13.52 GB    9.64 GB  linear  /
/dev/rhel/swap         rhel       1000.00 MB                             linear
/dev/lvm_pool/lvol001  lvm_pool    900.00 MB  xfs  896.67 MB  896.54 MB  linear
/dev/vda1                          500.00 MB  xfs  496.67 MB  403.56 MB  part    /boot
--------------------------------------------------------------------------------------------
The logical volume needs to be increased by another 500MB. To do so we will need to add an extra device to the pool:
~]# ssm resize -s +500M /dev/lvm_pool/lvol001 /dev/vdc
  Physical volume "/dev/vdc" successfully created
  Volume group "lvm_pool" successfully extended
Phase 1 - find and verify superblock...
Phase 2 - using internal log
        - scan filesystem freespace and inode maps...
        - found root inode chunk
Phase 3 - for each AG...
        - scan (but don't clear) agi unlinked lists...
        - process known inodes and perform inode discovery...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
        - process newly discovered inodes...
Phase 4 - check for duplicate blocks...
        - setting up duplicate extent list...
        - check for inodes claiming duplicate blocks...
        - agno = 0
        - agno = 1
        - agno = 2
        - agno = 3
No modify flag set, skipping phase 5
Phase 6 - check inode connectivity...
        - traversing filesystem ...
        - traversal finished ...
        - moving disconnected inodes to lost+found ...
Phase 7 - verify link counts...
No modify flag set, skipping filesystem flush and exiting.
  Extending logical volume lvol001 to 1.37 GiB
  Logical volume lvol001 successfully resized
meta-data=/dev/mapper/lvm_pool-lvol001 isize=256    agcount=4, agsize=57600 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=0
data     =                       bsize=4096   blocks=230400, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0 ftype=0
log      =internal               bsize=4096   blocks=853, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
data blocks changed from 230400 to 358400

SSM runs a check on the device and then extends the volume by the specified amount. This can be verified with the ssm list command.
# ssm list
------------------------------------------------------------------
Device          Free        Used      Total  Pool      Mount point
------------------------------------------------------------------
/dev/vda                           15.00 GB            PARTITIONED
/dev/vda1                         500.00 MB            /boot
/dev/vda2    0.00 KB    14.51 GB   14.51 GB  rhel
/dev/vdb     0.00 KB  1020.00 MB    1.00 GB  lvm_pool
/dev/vdc   640.00 MB   380.00 MB    1.00 GB  lvm_pool
------------------------------------------------------------------
------------------------------------------------------
Pool      Type  Devices       Free      Used     Total
------------------------------------------------------
lvm_pool  lvm   2        640.00 MB   1.37 GB   1.99 GB
rhel      lvm   1          0.00 KB  14.51 GB  14.51 GB
------------------------------------------------------
----------------------------------------------------------------------------------------------
Volume                 Pool      Volume size  FS      FS size        Free  Type    Mount point
----------------------------------------------------------------------------------------------
/dev/rhel/root         rhel         13.53 GB  xfs    13.52 GB     9.64 GB  linear  /
/dev/rhel/swap         rhel       1000.00 MB                               linear
/dev/lvm_pool/lvol001  lvm_pool      1.37 GB  xfs     1.36 GB     1.36 GB  linear
/dev/vda1                          500.00 MB  xfs   496.67 MB   403.56 MB  part    /boot
----------------------------------------------------------------------------------------------

Note

It is only possible to decrease an LVM volume's size; it is not supported with other volume types. This is done by using a - instead of a +. For example, to decrease the size of an LVM volume by 50M the command would be:
# ssm resize -s-50M /dev/lvm_pool/lvol002
  Rounding size to boundary between physical extents: 972.00 MiB
  WARNING: Reducing active logical volume to 972.00 MiB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce lvol002? [y/n]: y
  Reducing logical volume lvol002 to 972.00 MiB
  Logical volume lvol002 successfully resized
Without either the + or -, the value is taken as absolute.

16.2.6. Snapshot

To take a snapshot of an existing volume, use the ssm snapshot command.

Note

This operation fails if the back end that the volume belongs to does not support snapshotting.
To create a snapshot of the lvol001, use the following command:
# ssm snapshot /dev/lvm_pool/lvol001
  Logical volume "snap20150519T130900" created
To verify this, use the ssm list, and note the extra snapshot section.
# ssm list
----------------------------------------------------------------
Device        Free        Used      Total  Pool      Mount point
----------------------------------------------------------------
/dev/vda                         15.00 GB            PARTITIONED
/dev/vda1                       500.00 MB            /boot
/dev/vda2  0.00 KB    14.51 GB   14.51 GB  rhel
/dev/vdb   0.00 KB  1020.00 MB    1.00 GB  lvm_pool
/dev/vdc                          1.00 GB
----------------------------------------------------------------
--------------------------------------------------------
Pool      Type  Devices     Free        Used       Total
--------------------------------------------------------
lvm_pool  lvm   1        0.00 KB  1020.00 MB  1020.00 MB
rhel      lvm   1        0.00 KB    14.51 GB    14.51 GB
--------------------------------------------------------
----------------------------------------------------------------------------------------------
Volume                 Pool      Volume size  FS      FS size        Free  Type    Mount point
----------------------------------------------------------------------------------------------
/dev/rhel/root         rhel         13.53 GB  xfs    13.52 GB     9.64 GB  linear  /
/dev/rhel/swap         rhel       1000.00 MB                               linear
/dev/lvm_pool/lvol001  lvm_pool    900.00 MB  xfs   896.67 MB   896.54 MB  linear
/dev/vda1                          500.00 MB  xfs   496.67 MB   403.56 MB  part    /boot
----------------------------------------------------------------------------------------------
----------------------------------------------------------------------------------
Snapshot                           Origin   Pool      Volume size     Size  Type
----------------------------------------------------------------------------------
/dev/lvm_pool/snap20150519T130900  lvol001  lvm_pool    120.00 MB  0.00 KB  linear
----------------------------------------------------------------------------------

16.2.7. Removing an Item

The ssm remove is used to remove an item, either a device, pool, or volume.

Note

If a device is being used by a pool when removed, it will fail. This can be forced using the -f argument.
If the volume is mounted when removed, it will fail. Unlike the device, it cannot be forced with the -f argument.
To remove the lvm_pool and everything within it use the following command:
# ssm remove lvm_pool
Do you really want to remove volume group "lvm_pool" containing 2 logical volumes? [y/n]: y
Do you really want to remove active logical volume snap20150519T130900? [y/n]: y
  Logical volume "snap20150519T130900" successfully removed
Do you really want to remove active logical volume lvol001? [y/n]: y
  Logical volume "lvol001" successfully removed
  Volume group "lvm_pool" successfully removed

16.3. SSM Resources

For more information on SSM, see the following resources:

Chapter 17. Disk Quotas

Disk space can be restricted by implementing disk quotas which alert a system administrator before a user consumes too much disk space or a partition becomes full.
Disk quotas can be configured for individual users as well as user groups. This makes it possible to manage the space allocated for user-specific files (such as email) separately from the space allocated to the projects a user works on (assuming the projects are given their own groups).
In addition, quotas can be set not just to control the number of disk blocks consumed but to control the number of inodes (data structures that contain information about files in UNIX file systems). Because inodes are used to contain file-related information, this allows control over the number of files that can be created.
The quota RPM must be installed to implement disk quotas.

Note

This chapter is for all file systems, however some file systems have their own quota management tools. See the corresponding description for the applicable file systems.
For XFS file systems, see Section 3.3, “XFS Quota Management”.
Btrfs does not have disk quotas so is not covered.

17.1. Configuring Disk Quotas

To implement disk quotas, use the following steps:
  1. Enable quotas per file system by modifying the /etc/fstab file.
  2. Remount the file system(s).
  3. Create the quota database files and generate the disk usage table.
  4. Assign quota policies.
Each of these steps is discussed in detail in the following sections.

17.1.1. Enabling Quotas

Procedure 17.1. Enabling Quotas

  1. Log in as root.
  2. Edit the /etc/fstab file.
  3. Add either the usrquota or grpquota or both options to the file systems that require quotas.

Example 17.1. Edit /etc/fstab

For example, to use the text editor vim type the following:
# vim /etc/fstab

Example 17.2. Add Quotas

/dev/VolGroup00/LogVol00 /         ext3    defaults        1 1
LABEL=/boot              /boot     ext3    defaults        1 2
none                     /dev/pts  devpts  gid=5,mode=620  0 0
none                     /dev/shm  tmpfs   defaults        0 0
none                     /proc     proc    defaults        0 0
none                     /sys      sysfs   defaults        0 0
/dev/VolGroup00/LogVol02 /home     ext3    defaults,usrquota,grpquota  1 2
/dev/VolGroup00/LogVol01 swap      swap    defaults        0 0 . . .
In this example, the /home file system has both user and group quotas enabled.

Note

The following examples assume that a separate /home partition was created during the installation of Red Hat Enterprise Linux. The root (/) partition can be used for setting quota policies in the /etc/fstab file.

17.1.2. Remounting the File Systems

After adding either the usrquota or grpquota or both options, remount each file system whose fstab entry has been modified. If the file system is not in use by any process, use one of the following methods:
  • Run the umount command followed by the mount command to remount the file system. See the man page for both umount and mount for the specific syntax for mounting and unmounting various file system types.
  • Run the mount -o remount file-system command (where file-system is the name of the file system) to remount the file system. For example, to remount the /home file system, run the mount -o remount /home command.
If the file system is currently in use, the easiest method for remounting the file system is to reboot the system.

17.1.3. Creating the Quota Database Files

After each quota-enabled file system is remounted run the quotacheck command.
The quotacheck command examines quota-enabled file systems and builds a table of the current disk usage per file system. The table is then used to update the operating system's copy of disk usage. In addition, the file system's disk quota files are updated.

Note

The quotacheck command has no effect on XFS as the table of disk usage is completed automatically at mount time. See the man page xfs_quota(8) for more information.

Procedure 17.2. Creating the Quota Database Files

  1. Create the quota files on the file system using the following command:
    # quotacheck -cug /file system
  2. Generate the table of current disk usage per file system using the following command:
    # quotacheck -avug
Following are the options used to create quota files:
c
Specifies that the quota files should be created for each file system with quotas enable.
u
Checks for user quotas.
g
Checks for group quotas. If only -g is specified, only the group quota file is created.
If neither the -u or -g options are specified, only the user quota file is created.
The following options are used to generate the table of current disk usage:
a
Check all quota-enabled, locally-mounted file systems
v
Display verbose status information as the quota check proceeds
u
Check user disk quota information
g
Check group disk quota information
After quotacheck has finished running, the quota files corresponding to the enabled quotas (either user or group or both) are populated with data for each quota-enabled locally-mounted file system such as /home.

17.1.4. Assigning Quotas per User

The last step is assigning the disk quotas with the edquota command.
Prerequisite
  • User must exist prior to setting the user quota.

Procedure 17.3. Assigning Quotas per User

  1. To assign the quota for a user, use the following command:
    # edquota username
    Replace username with the user to which you want to assign the quotas.
  2. To verify that the quota for the user has been set, use the following command:
    # quota username

Example 17.3. Assigning Quotas to a user

For example, if a quota is enabled in /etc/fstab for the /home partition (/dev/VolGroup00/LogVol02 in the following example) and the command edquota testuser is executed, the following is shown in the editor configured as the default for the system:
Disk quotas for user testuser (uid 501):
		Filesystem                blocks     soft     hard    inodes   soft   hard
		/dev/VolGroup00/LogVol02  440436        0        0     37418      0      0

Note

The text editor defined by the EDITOR environment variable is used by edquota. To change the editor, set the EDITOR environment variable in your ~/.bash_profile file to the full path of the editor of your choice.
The first column is the name of the file system that has a quota enabled for it. The second column shows how many blocks the user is currently using. The next two columns are used to set soft and hard block limits for the user on the file system. The inodes column shows how many inodes the user is currently using. The last two columns are used to set the soft and hard inode limits for the user on the file system.
The hard block limit is the absolute maximum amount of disk space that a user or group can use. Once this limit is reached, no further disk space can be used.
The soft block limit defines the maximum amount of disk space that can be used. However, unlike the hard limit, the soft limit can be exceeded for a certain amount of time. That time is known as the grace period. The grace period can be expressed in seconds, minutes, hours, days, weeks, or months.
If any of the values are set to 0, that limit is not set. In the text editor, change the desired limits.

Example 17.4. Change Desired Limits

For example:
Disk quotas for user testuser (uid 501):
Filesystem                blocks     soft     hard   inodes   soft   hard
/dev/VolGroup00/LogVol02  440436   500000   550000    37418      0      0
To verify that the quota for the user has been set, use the command:
# quota testuser
Disk quotas for user username (uid 501):
   Filesystem  blocks   quota   limit   grace   files   quota   limit   grace
     /dev/sdb    1000*   1000    1000               0       0       0

17.1.5. Assigning Quotas per Group

Quotas can also be assigned on a per-group basis.
Prerequisite
  • Group must exist prior to setting the group quota.

Procedure 17.4. Assigning Quotas per Group

  1. To set a group quota, use the following command:
    # edquota -g groupname
  2. To verify that the group quota is set, use the following command:
    # quota -g groupname

Example 17.5. Assigning quotas to group

For example, to set a group quota for the devel group, use the command:
# edquota -g devel
This command displays the existing quota for the group in the text editor:
Disk quotas for group devel (gid 505):
Filesystem                blocks    soft     hard    inodes    soft    hard
/dev/VolGroup00/LogVol02  440400       0        0     37418       0       0
Modify the limits, then save the file.
To verify that the group quota has been set, use the command:
# quota -g devel

17.1.6. Setting the Grace Period for Soft Limits

If a given quota has soft limits, you can edit the grace period (i.e. the amount of time a soft limit can be exceeded) with the following command:
# edquota -t
This command works on quotas for inodes or blocks, for either users or groups.

Important

While other edquota commands operate on quotas for a particular user or group, the -t option operates on every file system with quotas enabled.

17.2. Managing Disk Quotas

If quotas are implemented, they need some maintenance mostly in the form of watching to see if the quotas are exceeded and making sure the quotas are accurate.
If users repeatedly exceed their quotas or consistently reach their soft limits, a system administrator has a few choices to make depending on what type of users they are and how much disk space impacts their work. The administrator can either help the user determine how to use less disk space or increase the user's disk quota.

17.2.1. Enabling and Disabling

It is possible to disable quotas without setting them to 0. To turn all user and group quotas off, use the following command:
# quotaoff -vaug
If neither the -u or -g options are specified, only the user quotas are disabled. If only -g is specified, only group quotas are disabled. The -v switch causes verbose status information to display as the command executes.
To enable user and group quotas again, use the following command:
# quotaon
To enable user and group quotas for all file systems, use the following command:
# quotaon -vaug
If neither the -u or -g options are specified, only the user quotas are enabled. If only -g is specified, only group quotas are enabled.
To enable quotas for a specific file system, such as /home, use the following command:
# quotaon -vug /home

Note

The quotaon command is not always needed for XFS because it is performed automatically at mount time. Refer to the man page quotaon(8) for more information.

17.2.2. Reporting on Disk Quotas

Creating a disk usage report entails running the repquota utility.

Example 17.6. Output of the repquota Command

For example, the command repquota /home produces this output:
*** Report for user quotas on device /dev/mapper/VolGroup00-LogVol02
Block grace time: 7days; Inode grace time: 7days
			Block limits			File limits
User		used	soft	hard	grace	used	soft	hard	grace
----------------------------------------------------------------------
root      --      36       0       0              4     0     0
kristin   --     540       0       0            125     0     0
testuser  --  440400  500000  550000          37418     0     0
To view the disk usage report for all (option -a) quota-enabled file systems, use the command:
# repquota -a
While the report is easy to read, a few points should be explained. The -- displayed after each user is a quick way to determine whether the block or inode limits have been exceeded. If either soft limit is exceeded, a + appears in place of the corresponding -; the first - represents the block limit, and the second represents the inode limit.
The grace columns are normally blank. If a soft limit has been exceeded, the column contains a time specification equal to the amount of time remaining on the grace period. If the grace period has expired, none appears in its place.

17.2.3. Keeping Quotas Accurate

When a file system fails to unmount cleanly, for example due to a system crash, it is necessary to run the following command:
# quotacheck
However, quotacheck can be run on a regular basis, even if the system has not crashed. Safe methods for periodically running quotacheck include:
Ensuring quotacheck runs on next reboot

Note

This method works best for (busy) multiuser systems which are periodically rebooted.
Save a shell script into the /etc/cron.daily/ or /etc/cron.weekly/ directory or schedule one using the following command:
# crontab -e
The crontab -e command contains the touch /forcequotacheck command. This creates an empty forcequotacheck file in the root directory, which the system init script looks for at boot time. If it is found, the init script runs quotacheck. Afterward, the init script removes the /forcequotacheck file; thus, scheduling this file to be created periodically with cron ensures that quotacheck is run during the next reboot.
For more information about cron, see man cron.
Running quotacheck in single user mode
An alternative way to safely run quotacheck is to boot the system into single-user mode to prevent the possibility of data corruption in quota files and run the following commands:
# quotaoff -vug /file_system
# quotacheck -vug /file_system
# quotaon -vug /file_system
Running quotacheck on a running system
If necessary, it is possible to run quotacheck on a machine during a time when no users are logged in, and thus have no open files on the file system being checked. Run the command quotacheck -vug file_system; this command will fail if quotacheck cannot remount the given file_system as read-only. Note that, following the check, the file system will be remounted read-write.

Warning

Running quotacheck on a live file system mounted read-write is not recommended due to the possibility of quota file corruption.
See man cron for more information about configuring cron.

17.3. Disk Quota References

For more information on disk quotas, refer to the man pages of the following commands:
  • quotacheck
  • edquota
  • repquota
  • quota
  • quotaon
  • quotaoff

Chapter 18. Redundant Array of Independent Disks (RAID)

The basic idea behind RAID is to combine multiple small, inexpensive disk drives into an array to accomplish performance or redundancy goals not attainable with one large and expensive drive. This array of drives appears to the computer as a single logical storage unit or drive.
RAID allows information to be spread across several disks. RAID uses techniques such as disk striping (RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from hard disk crashes.
RAID distributes data across each drive in the array by breaking it down into consistently-sized chunks (commonly 256K or 512k, although other values are acceptable). Each chunk is then written to a hard drive in the RAID array according to the RAID level employed. When the data is read, the process is reversed, giving the illusion that the multiple drives in the array are actually one large drive.
System Administrators and others who manage large amounts of data would benefit from using RAID technology. Primary reasons to deploy RAID include:
  • Enhances speed
  • Increases storage capacity using a single virtual disk
  • Minimizes data loss from disk failure

18.1. RAID Types

There are three possible RAID approaches: Firmware RAID, Hardware RAID, and Software RAID.

Firmware RAID

Firmware RAID, also known as ATARAID, is a type of software RAID where the RAID sets can be configured using a firmware-based menu. The firmware used by this type of RAID also hooks into the BIOS, allowing you to boot from its RAID sets. Different vendors use different on-disk metadata formats to mark the RAID set members. The Intel Matrix RAID is a good example of a firmware RAID system.

Hardware RAID

The hardware-based array manages the RAID subsystem independently from the host. It presents a single disk per RAID array to the host.
A Hardware RAID device may be internal or external to the system, with internal devices commonly consisting of a specialized controller card that handles the RAID tasks transparently to the operating system and with external devices commonly connecting to the system via SCSI, Fibre Channel, iSCSI, InfiniBand, or other high speed network interconnect and presenting logical volumes to the system.
RAID controller cards function like a SCSI controller to the operating system, and handle all the actual drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI controller) and then adds them to the RAID controllers configuration. The operating system will not be able to tell the difference.

Software RAID

Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis [2] are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With today's faster CPUs, Software RAID also generally outperforms Hardware RAID.
The Linux kernel contains a multi-disk (MD) driver that allows the RAID solution to be completely hardware independent. The performance of a software-based array depends on the server CPU performance and load.
Key features of the Linux software RAID stack:
  • Multithreaded design
  • Portability of arrays between Linux machines without reconstruction
  • Backgrounded array reconstruction using idle system resources
  • Hot-swappable drive support
  • Automatic CPU detection to take advantage of certain CPU features such as streaming SIMD support
  • Automatic correction of bad sectors on disks in an array
  • Regular consistency checks of RAID data to ensure the health of the array
  • Proactive monitoring of arrays with email alerts sent to a designated email address on important events
  • Write-intent bitmaps which drastically increase the speed of resync events by allowing the kernel to know precisely which portions of a disk need to be resynced instead of having to resync the entire array
  • Resync checkpointing so that if you reboot your computer during a resync, at startup the resync will pick up where it left off and not start all over again
  • The ability to change parameters of the array after installation. For example, you can grow a 4-disk RAID5 array to a 5-disk RAID5 array when you have a new disk to add. This grow operation is done live and does not require you to reinstall on the new array.

18.2. RAID Levels and Linear Support

RAID supports various configurations, including levels 0, 1, 4, 5, 6, 10, and linear. These RAID types are defined as follows:
Level 0
RAID level 0, often called "striping," is a performance-oriented striped data mapping technique. This means the data being written to the array is broken down into strips and written across the member disks of the array, allowing high I/O performance at low inherent cost but provides no redundancy.
Many RAID level 0 implementations will only stripe the data across the member devices up to the size of the smallest device in the array. This means that if you have multiple devices with slightly different sizes, each device will get treated as though it is the same size as the smallest drive. Therefore, the common storage capacity of a level 0 array is equal to the capacity of the smallest member disk in a Hardware RAID or the capacity of smallest member partition in a Software RAID multiplied by the number of disks or partitions in the array.
Level 1
RAID level 1, or "mirroring," has been used longer than any other form of RAID. Level 1 provides redundancy by writing identical data to each member disk of the array, leaving a "mirrored" copy on each disk. Mirroring remains popular due to its simplicity and high level of data availability. Level 1 operates with two or more disks, and provides very good data reliability and improves performance for read-intensive applications but at a relatively high cost. [3]
The storage capacity of the level 1 array is equal to the capacity of the smallest mirrored hard disk in a Hardware RAID or the smallest mirrored partition in a Software RAID. Level 1 redundancy is the highest possible among all RAID types, with the array being able to operate with only a single disk present.
Level 4
Level 4 uses parity [4] concentrated on a single disk drive to protect data. Because the dedicated parity disk represents an inherent bottleneck on all write transactions to the RAID array, level 4 is seldom used without accompanying technologies such as write-back caching, or in specific circumstances where the system administrator is intentionally designing the software RAID device with this bottleneck in mind (such as an array that will have little to no write transactions once the array is populated with data). RAID level 4 is so rarely used that it is not available as an option in Anaconda. However, it could be created manually by the user if truly needed.
The storage capacity of Hardware RAID level 4 is equal to the capacity of the smallest member partition multiplied by the number of partitions minus one. Performance of a RAID level 4 array will always be asymmetrical, meaning reads will outperform writes. This is because writes consume extra CPU and main memory bandwidth when generating parity, and then also consume extra bus bandwidth when writing the actual data to disks because you are writing not only the data, but also the parity. Reads need only read the data and not the parity unless the array is in a degraded state. As a result, reads generate less traffic to the drives and across the busses of the computer for the same amount of data transfer under normal operating conditions.
Level 5
This is the most common type of RAID. By distributing parity across all of an array's member disk drives, RAID level 5 eliminates the write bottleneck inherent in level 4. The only performance bottleneck is the parity calculation process itself. With modern CPUs and Software RAID, that is usually not a bottleneck at all since modern CPUs can generate parity very fast. However, if you have a sufficiently large number of member devices in a software RAID5 array such that the combined aggregate data transfer speed across all devices is high enough, then this bottleneck can start to come into play.
As with level 4, level 5 has asymmetrical performance, with reads substantially outperforming writes. The storage capacity of RAID level 5 is calculated the same way as with level 4.
Level 6
This is a common level of RAID when data redundancy and preservation, and not performance, are the paramount concerns, but where the space inefficiency of level 1 is not acceptable. Level 6 uses a complex parity scheme to be able to recover from the loss of any two drives in the array. This complex parity scheme creates a significantly higher CPU burden on software RAID devices and also imposes an increased burden during write transactions. As such, level 6 is considerably more asymmetrical in performance than levels 4 and 5.
The total capacity of a RAID level 6 array is calculated similarly to RAID level 5 and 4, except that you must subtract 2 devices (instead of 1) from the device count for the extra parity storage space.
Level 10
This RAID level attempts to combine the performance advantages of level 0 with the redundancy of level 1. It also helps to alleviate some of the space wasted in level 1 arrays with more than 2 devices. With level 10, it is possible to create a 3-drive array configured to store only 2 copies of each piece of data, which then allows the overall array size to be 1.5 times the size of the smallest devices instead of only equal to the smallest device (like it would be with a 3-device, level 1 array).
The number of options available when creating level 10 arrays as well as the complexity of selecting the right options for a specific use case make it impractical to create during installation. It is possible to create one manually using the command line mdadm tool. For more information on the options and their respective performance trade-offs, see man md.
Linear RAID
Linear RAID is a grouping of drives to create a larger virtual drive. In linear RAID, the chunks are allocated sequentially from one member drive, going to the next drive only when the first is completely filled. This grouping provides no performance benefit, as it is unlikely that any I/O operations split between member drives. Linear RAID also offers no redundancy and decreases reliability; if any one member drive fails, the entire array cannot be used. The capacity is the total of all member disks.

18.3. Linux RAID Subsystems

RAID in Linux is composed of the following subsystems:

Linux Hardware RAID Controller Drivers

Hardware RAID controllers have no specific RAID subsystem in Linux. Because they use special RAID chipsets, hardware RAID controllers come with their own drivers; these drivers allow the system to detect the RAID sets as regular disks.

mdraid

The mdraid subsystem was designed as a software RAID solution for Linux; it is also the preferred solution for software RAID under Linux. This subsystem uses its own metadata format, generally referred to as native mdraid metadata.
mdraid also supports other metadata formats, known as external metadata. Red Hat Enterprise Linux 7 uses mdraid with external metadata to access ISW / IMSM (Intel firmware RAID) sets. mdraid sets are configured and controlled through the mdadm utility.

dmraid

The dmraid tool is used on a wide variety of firmware RAID implementations. dmraid also supports Intel firmware RAID, although Red Hat Enterprise Linux 7 uses mdraid to access Intel firmware RAID sets.

Note

dmraid has been deprecated since the Red Hat Enterprise Linux 7.5 release. It will be removed in a future major release of Red Hat Enterprise Linux. For more information, see Deprecated Functionality in the Red Hat Enterprise Linux 7.5 Release Notes.

18.4. RAID Support in the Anaconda Installer

The Anaconda installer automatically detects any hardware and firmware RAID sets on a system, making them available for installation. Anaconda also supports software RAID using mdraid, and can recognize existing mdraid sets.
Anaconda provides utilities for creating RAID sets during installation; however, these utilities only allow partitions (as opposed to entire disks) to be members of new sets. To use an entire disk for a set, create a partition on it spanning the entire disk, and use that partition as the RAID set member.
When the root file system uses a RAID set, Anaconda adds special kernel command-line options to the bootloader configuration telling the initrd which RAID set(s) to activate before searching for the root file system.
For instructions on configuring RAID during installation, see the Red Hat Enterprise Linux 7 Installation Guide.

18.5. Converting Root Disk to RAID1 after Installation

If you need to convert a non-raided root disk to a RAID1 mirror after installing Red Hat Enterprise Linux 7, see the instructions in the following Red Hat Knowledgebase article: How do I convert my root disk to RAID1 after installation of Red Hat Enterprise Linux 7?
On the PowerPC (PPC) architecture, take the following additional steps:
  1. Copy the contents of the PowerPC Reference Platform (PReP) boot partition from /dev/sda1 to /dev/sdb1:
    # dd if=/dev/sda1 of=/dev/sdb1 
  2. Update the Prep and boot flag on the first partition on both disks:
    $ parted /dev/sda set 1 prep on
    $ parted /dev/sda set 1 boot on
    
    $ parted /dev/sdb set 1 prep on
    $ parted /dev/sdb set 1 boot on 

Note

Running the grub2-install /dev/sda command does not work on a PowerPC machine and returns an error, but the system boots as expected.

18.6. Configuring RAID Sets

Most RAID sets are configured during creation, typically through the firmware menu or from the installer. In some cases, you may need to create or modify RAID sets after installing the system, preferably without having to reboot the machine and enter the firmware menu to do so.
Some hardware RAID controllers allow you to configure RAID sets on-the-fly or even define completely new sets after adding extra disks. This requires the use of driver-specific utilities, as there is no standard API for this. For more information, see your hardware RAID controller's driver documentation for information on this.

mdadm

The mdadm command-line tool is used to manage software RAID in Linux, i.e. mdraid. For information on the different mdadm modes and options, see man mdadm. The man page also contains useful examples for common operations like creating, monitoring, and assembling software RAID arrays.

dmraid

As the name suggests, dmraid is used to manage device-mapper RAID sets. The dmraid tool finds ATARAID devices using multiple metadata format handlers, each supporting various formats. For a complete list of supported formats, run dmraid -l.
As mentioned earlier in Section 18.3, “Linux RAID Subsystems”, the dmraid tool cannot configure RAID sets after creation. For more information about using dmraid, see man dmraid.

18.7. Creating Advanced RAID Devices

In some cases, you may wish to install the operating system on an array that can't be created after the installation completes. Usually, this means setting up the /boot or root file system arrays on a complex RAID device; in such cases, you may need to use array options that are not supported by Anaconda. To work around this, perform the following procedure:

Procedure 18.1. Creating Advanced RAID Devices

  1. Insert the install disk.
  2. During the initial boot up, select Rescue Mode instead of Install or Upgrade. When the system fully boots into Rescue mode, the user will be presented with a command line terminal.
  3. From this terminal, use parted to create RAID partitions on the target hard drives. Then, use mdadm to manually create raid arrays from those partitions using any and all settings and options available. For more information on how to do these, see Chapter 13, Partitions, man parted, and man mdadm.
  4. Once the arrays are created, you can optionally create file systems on the arrays as well.
  5. Reboot the computer and this time select Install or Upgrade to install as normal. As Anaconda searches the disks in the system, it will find the pre-existing RAID devices.
  6. When asked about how to use the disks in the system, select Custom Layout and click Next. In the device listing, the pre-existing MD RAID devices will be listed.
  7. Select a RAID device, click Edit and configure its mount point and (optionally) the type of file system it should use (if you did not create one earlier) then click Done. Anaconda will perform the install to this pre-existing RAID device, preserving the custom options you selected when you created it in Rescue Mode.

Note

The limited Rescue Mode of the installer does not include man pages. Both the man mdadm and man md contain useful information for creating custom RAID arrays, and may be needed throughout the workaround. As such, it can be helpful to either have access to a machine with these man pages present, or to print them out prior to booting into Rescue Mode and creating your custom arrays.


[2] A hot-swap chassis allows you to remove a hard drive without having to power-down your system.
[3] RAID level 1 comes at a high cost because you write the same information to all of the disks in the array, provides data reliability, but in a much less space-efficient manner than parity based RAID levels such as level 5. However, this space inefficiency comes with a performance benefit: parity-based RAID levels consume considerably more CPU power in order to generate the parity while RAID level 1 simply writes the same data more than once to the multiple RAID members with very little CPU overhead. As such, RAID level 1 can outperform the parity-based RAID levels on machines where software RAID is employed and CPU resources on the machine are consistently taxed with operations other than RAID activities.
[4] Parity information is calculated based on the contents of the rest of the member disks in the array. This information can then be used to reconstruct data when one disk in the array fails. The reconstructed data can then be used to satisfy I/O requests to the failed disk before it is replaced and to repopulate the failed disk after it has been replaced.

Chapter 19. Using the mount Command

On Linux, UNIX, and similar operating systems, file systems on different partitions and removable devices (CDs, DVDs, or USB flash drives for example) can be attached to a certain point (the mount point) in the directory tree, and then detached again. To attach or detach a file system, use the mount or umount command respectively. This chapter describes the basic use of these commands, as well as some advanced topics, such as moving a mount point or creating shared subtrees.

19.1. Listing Currently Mounted File Systems

To display all currently attached file systems, use the following command with no additional arguments:
$ mount
This command displays the list of known mount points. Each line provides important information about the device name, the file system type, the directory in which it is mounted, and relevant mount options in the following form:
device on directory type type (options)
The findmnt utility, which allows users to list mounted file systems in a tree-like form, is also available from Red Hat Enterprise Linux 6.1. To display all currently attached file systems, run the findmnt command with no additional arguments:
$ findmnt

19.1.1. Specifying the File System Type

By default, the output of the mount command includes various virtual file systems such as sysfs and tmpfs. To display only the devices with a certain file system type, provide the -t option:
$ mount -t type
Similarly, to display only the devices with a certain file system using the findmnt command:
$ findmnt -t type
For a list of common file system types, see Table 19.1, “Common File System Types”. For an example usage, see Example 19.1, “Listing Currently Mounted ext4 File Systems”.

Example 19.1. Listing Currently Mounted ext4 File Systems

Usually, both / and /boot partitions are formatted to use ext4. To display only the mount points that use this file system, use the following command:
$ mount -t ext4
/dev/sda2 on / type ext4 (rw)
/dev/sda1 on /boot type ext4 (rw)
To list such mount points using the findmnt command, type:
$ findmnt -t ext4
TARGET SOURCE    FSTYPE OPTIONS
/      /dev/sda2 ext4   rw,realtime,seclabel,barrier=1,data=ordered
/boot  /dev/sda1 ext4   rw,realtime,seclabel,barrier=1,data=ordered

19.2. Mounting a File System

To attach a certain file system, use the mount command in the following form:
$ mount [option] device directory
The device can be identified by:
  • a full path to a block device: for example, /dev/sda3
  • a universally unique identifier (UUID): for example, UUID=34795a28-ca6d-4fd8-a347-73671d0c19cb
  • a volume label: for example, LABEL=home
Note that while a file system is mounted, the original content of the directory is not accessible.

Important

Linux does not prevent a user from mounting a file system to a directory with a file system already attached to it. To determine whether a particular directory serves as a mount point, run the findmnt utility with the directory as its argument and verify the exit code:
findmnt directory; echo $?
If no file system is attached to the directory, the given command returns 1.
When you run the mount command without all required information, that is without the device name, the target directory, or the file system type, the mount reads the contents of the /etc/fstab file to check if the given file system is listed. The /etc/fstab file contains a list of device names and the directories in which the selected file systems are set to be mounted as well as the file system type and mount options. Therefore, when mounting a file system that is specified in /etc/fstab, you can choose one of the following options:
mount [option] directory
mount [option] device
Note that permissions are required to mount the file systems unless the command is run as root (see Section 19.2.2, “Specifying the Mount Options”).

Note

To determine the UUID and—if the device uses it—the label of a particular device, use the blkid command in the following form:
blkid device
For example, to display information about /dev/sda3:
# blkid /dev/sda3
/dev/sda3: LABEL="home" UUID="34795a28-ca6d-4fd8-a347-73671d0c19cb" TYPE="ext3"

19.2.1. Specifying the File System Type

In most cases, mount detects the file system automatically. However, there are certain file systems, such as NFS (Network File System) or CIFS (Common Internet File System), that are not recognized, and need to be specified manually. To specify the file system type, use the mount command in the following form:
$ mount -t type device directory
Table 19.1, “Common File System Types” provides a list of common file system types that can be used with the mount command. For a complete list of all available file system types, see the section called “Manual Page Documentation”.
Table 19.1. Common File System Types
Type Description
ext2 The ext2 file system.
ext3 The ext3 file system.
ext4 The ext4 file system.
btrfs The btrfs file system.
xfs The xfs file system.
iso9660 The ISO 9660 file system. It is commonly used by optical media, typically CDs.
nfs The NFS file system. It is commonly used to access files over the network.
nfs4 The NFSv4 file system. It is commonly used to access files over the network.
udf The UDF file system. It is commonly used by optical media, typically DVDs.
vfat The FAT file system. It is commonly used on machines that are running the Windows operating system, and on certain digital media such as USB flash drives or floppy disks.

Example 19.2. Mounting a USB Flash Drive

Older USB flash drives often use the FAT file system. Assuming that such drive uses the /dev/sdc1 device and that the /media/flashdisk/ directory exists, mount it to this directory by typing the following at a shell prompt as root:
~]# mount -t vfat /dev/sdc1 /media/flashdisk

19.2.2. Specifying the Mount Options

To specify additional mount options, use the command in the following form:
mount -o options device directory
When supplying multiple options, do not insert a space after a comma, or mount interprets incorrectly the values following spaces as additional parameters.
Table 19.2, “Common Mount Options” provides a list of common mount options. For a complete list of all available options, consult the relevant manual page as referred to in the section called “Manual Page Documentation”.
Table 19.2. Common Mount Options
Option Description
async Allows the asynchronous input/output operations on the file system.
auto Allows the file system to be mounted automatically using the mount -a command.
defaults Provides an alias for async,auto,dev,exec,nouser,rw,suid.
exec Allows the execution of binary files on the particular file system.
loop Mounts an image as a loop device.
noauto Default behavior disallows the automatic mount of the file system using the mount -a command.
noexec Disallows the execution of binary files on the particular file system.
nouser Disallows an ordinary user (that is, other than root) to mount and unmount the file system.
remount Remounts the file system in case it is already mounted.
ro Mounts the file system for reading only.
rw Mounts the file system for both reading and writing.
user Allows an ordinary user (that is, other than root) to mount and unmount the file system.
See Example 19.3, “Mounting an ISO Image” for an example usage.

Example 19.3. Mounting an ISO Image

An ISO image (or a disk image in general) can be mounted by using the loop device. Assuming that the ISO image of the Fedora 14 installation disc is present in the current working directory and that the /media/cdrom/ directory exists, mount the image to this directory by running the following command:
# mount -o ro,loop Fedora-14-x86_64-Live-Desktop.iso /media/cdrom
Note that ISO 9660 is by design a read-only file system.

19.2.3. Sharing Mounts

Occasionally, certain system administration tasks require access to the same file system from more than one place in the directory tree (for example, when preparing a chroot environment). This is possible, and Linux allows you to mount the same file system to as many directories as necessary. Additionally, the mount command implements the --bind option that provides a means for duplicating certain mounts. Its usage is as follows:
$ mount --bind old_directory new_directory
Although this command allows a user to access the file system from both places, it does not apply on the file systems that are mounted within the original directory. To include these mounts as well, use the following command:
$ mount --rbind old_directory new_directory
Additionally, to provide as much flexibility as possible, Red Hat Enterprise Linux 7 implements the functionality known as shared subtrees. This feature allows the use of the following four mount types:
Shared Mount
A shared mount allows the creation of an exact replica of a given mount point. When a mount point is marked as a shared mount, any mount within the original mount point is reflected in it, and vice versa. To change the type of a mount point to a shared mount, type the following at a shell prompt:
$ mount --make-shared mount_point
Alternatively, to change the mount type for the selected mount point and all mount points under it:
$ mount --make-rshared mount_point

Example 19.4. Creating a Shared Mount Point

There are two places where other file systems are commonly mounted: the /media/ directory for removable media, and the /mnt/ directory for temporarily mounted file systems. By using a shared mount, you can make these two directories share the same content. To do so, as root, mark the /media/ directory as shared:
# mount --bind /media /media
# mount --make-shared /media
Create its duplicate in /mnt/ by using the following command:
# mount --bind /media /mnt
It is now possible to verify that a mount within /media/ also appears in /mnt/. For example, if the CD-ROM drive contains non-empty media and the /media/cdrom/ directory exists, run the following commands:
# mount /dev/cdrom /media/cdrom
# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
Similarly, it is possible to verify that any file system mounted in the /mnt/ directory is reflected in /media/. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type:
# # mount /dev/sdc1 /mnt/flashdisk
# ls /media/flashdisk
en-US  publican.cfg
# ls /mnt/flashdisk
en-US  publican.cfg
Slave Mount
A slave mount allows the creation of a limited duplicate of a given mount point. When a mount point is marked as a slave mount, any mount within the original mount point is reflected in it, but no mount within a slave mount is reflected in its original. To change the type of a mount point to a slave mount, type the following at a shell prompt:
mount --make-slave mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it by typing:
mount --make-rslave mount_point

Example 19.5. Creating a Slave Mount Point

This example shows how to get the content of the /media/ directory to appear in /mnt/ as well, but without any mounts in the /mnt/ directory to be reflected in /media/. As root, first mark the /media/ directory as shared:
~]# mount --bind /media /media
~]# mount --make-shared /media
Then create its duplicate in /mnt/, but mark it as "slave":
~]# mount --bind /media /mnt
~]# mount --make-slave /mnt
Now verify that a mount within /media/ also appears in /mnt/. For example, if the CD-ROM drive contains non-empty media and the /media/cdrom/ directory exists, run the following commands:
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
EFI  GPL  isolinux  LiveOS
Also verify that file systems mounted in the /mnt/ directory are not reflected in /media/. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type:
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
Private Mount
A private mount is the default type of mount, and unlike a shared or slave mount, it does not receive or forward any propagation events. To explicitly mark a mount point as a private mount, type the following at a shell prompt:
mount --make-private mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it:
mount --make-rprivate mount_point

Example 19.6. Creating a Private Mount Point

Taking into account the scenario in Example 19.4, “Creating a Shared Mount Point”, assume that a shared mount point has been previously created by using the following commands as root:
~]# mount --bind /media /media
~]# mount --make-shared /media
~]# mount --bind /media /mnt
To mark the /mnt/ directory as private, type:
~]# mount --make-private /mnt
It is now possible to verify that none of the mounts within /media/ appears in /mnt/. For example, if the CD-ROM drives contains non-empty media and the /media/cdrom/ directory exists, run the following commands:
~]# mount /dev/cdrom /media/cdrom
~]# ls /media/cdrom
EFI  GPL  isolinux  LiveOS
~]# ls /mnt/cdrom
~]#
It is also possible to verify that file systems mounted in the /mnt/ directory are not reflected in /media/. For instance, if a non-empty USB flash drive that uses the /dev/sdc1 device is plugged in and the /mnt/flashdisk/ directory is present, type:
~]# mount /dev/sdc1 /mnt/flashdisk
~]# ls /media/flashdisk
~]# ls /mnt/flashdisk
en-US  publican.cfg
Unbindable Mount
In order to prevent a given mount point from being duplicated whatsoever, an unbindable mount is used. To change the type of a mount point to an unbindable mount, type the following at a shell prompt:
mount --make-unbindable mount_point
Alternatively, it is possible to change the mount type for the selected mount point and all mount points under it:
mount --make-runbindable mount_point

Example 19.7. Creating an Unbindable Mount Point

To prevent the /media/ directory from being shared, as root:
# mount --bind /media /media
# mount --make-unbindable /media
This way, any subsequent attempt to make a duplicate of this mount fails with an error:
# mount --bind /media /mnt
mount: wrong fs type, bad option, bad superblock on /media,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail  or so

19.2.4. Moving a Mount Point

To change the directory in which a file system is mounted, use the following command:
# mount --move old_directory new_directory

Example 19.8. Moving an Existing NFS Mount Point

An NFS storage contains user directories and is already mounted in /mnt/userdirs/. As root, move this mount point to /home by using the following command:
# mount --move /mnt/userdirs /home
To verify the mount point has been moved, list the content of both directories:
# ls /mnt/userdirs
# ls /home
jill  joe

19.2.5. Setting Read-only Permissions for root

Sometimes, you need to mount the root file system with read-only permissions. Example use cases include enhancing security or ensuring data integrity after an unexpected system power-off.
19.2.5.1. Configuring root to Mount with Read-only Permissions on Boot
  1. In the /etc/sysconfig/readonly-root file, change READONLY to yes:
    # Set to 'yes' to mount the file systems as read-only.
    READONLY=yes
    [output truncated]
  2. Change defaults to ro in the root entry (/) in the /etc/fstab file:
    /dev/mapper/luks-c376919e... / ext4 ro,x-systemd.device-timeout=0 1 1
  3. Add ro to the GRUB_CMDLINE_LINUX directive in the /etc/default/grub file and ensure that it does not contain rw:
    GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=rhel/root rd.lvm.lv=rhel/swap rhgb quiet ro"
  4. Recreate the GRUB2 configuration file:
    # grub2-mkconfig -o /boot/grub2/grub.cfg
  5. If you need to add files and directories to be mounted with write permissions in the tmpfs file system, create a text file in the /etc/rwtab.d/ directory and put the configuration there. For example, to mount /etc/example/file with write permissions, add this line to the /etc/rwtab.d/example file:
    files /etc/example/file

    Important

    Changes made to files and directories in tmpfs do not persist across boots.
  6. Reboot the system.
19.2.5.2. Remounting root Instantly
If root (/) was mounted with read-only permissions on system boot, you can remount it with write permissions:
# mount -o remount,rw /
This can be particularly useful when / is incorrectly mounted with read-only permissions.
To remount / with read-only permissions again, run:
# mount -o remount,ro /

Note

This command mounts the whole / with read-only permissions. A better approach is to retain write permissions for certain files and directories by copying them into RAM, as described in Section 19.2.5.1, “Configuring root to Mount with Read-only Permissions on Boot”.
19.2.5.3. Files and Directories That Retain Write Permissions
For the system to function properly, some files and directories need to retain write permissions. With root in read-only mode, they are mounted in RAM in the tmpfs temporary file system. The default set of such files and directories is read from the /etc/rwtab file, which contains:
dirs	/var/cache/man
dirs	/var/gdm
[output truncated]
empty	/tmp
empty	/var/cache/foomatic
[output truncated]
files	/etc/adjtime
files	/etc/ntp.conf
[output truncated]
Entries in the /etc/rwtab file follow this format:
how the file or directory is copied to tmpfs       	path to the file or directory
A file or directory can be copied to tmpfs in the following three ways:
  • empty path: An empty path is copied to tmpfs. Example: empty /tmp
  • dirs path: A directory tree is copied to tmpfs, empty. Example: dirs /var/run
  • files path: A file or a directory tree is copied to tmpfs intact. Example: files /etc/resolv.conf
The same format applies when adding custom paths to /etc/rwtab.d/.

19.3. Unmounting a File System

To detach a previously mounted file system, use either of the following variants of the umount command:
$ umount directory
$ umount device
Note that unless this is performed while logged in as root, the correct permissions must be available to unmount the file system. For more information, see Section 19.2.2, “Specifying the Mount Options”. See Example 19.9, “Unmounting a CD” for an example usage.

Important

When a file system is in use (for example, when a process is reading a file on this file system, or when it is used by the kernel), running the umount command fails with an error. To determine which processes are accessing the file system, use the fuser command in the following form:
$ fuser -m directory
For example, to list the processes that are accessing a file system mounted to the /media/cdrom/ directory:
$ fuser -m /media/cdrom
/media/cdrom:         1793  2013  2022  2435 10532c 10672c

Example 19.9. Unmounting a CD

To unmount a CD that was previously mounted to the /media/cdrom/ directory, use the following command:
$ umount /media/cdrom

19.4. mount Command References

The following resources provide an in-depth documentation on the subject.

Manual Page Documentation

  • man 8 mount: The manual page for the mount command that provides a full documentation on its usage.
  • man 8 umount: The manual page for the umount command that provides a full documentation on its usage.
  • man 8 findmnt: The manual page for the findmnt command that provides a full documentation on its usage.
  • man 5 fstab: The manual page providing a thorough description of the /etc/fstab file format.

Useful Websites

  • Shared subtrees — An LWN article covering the concept of shared subtrees.

Chapter 20. The volume_key Function

The volume_key function provides two tools, libvolume_key and volume_key. libvolume_key is a library for manipulating storage volume encryption keys and storing them separately from volumes. volume_key is an associated command line tool used to extract keys and passphrases in order to restore access to an encrypted hard drive.
This is useful for when the primary user forgets their keys and passwords, after an employee leaves abruptly, or in order to extract data after a hardware or software failure corrupts the header of the encrypted volume. In a corporate setting, the IT help desk can use volume_key to back up the encryption keys before handing over the computer to the end user.
Currently, volume_key only supports the LUKS volume encryption format.

Note

volume_key is not included in a standard install of Red Hat Enterprise Linux 7 server. For information on installing it, refer to http://fedoraproject.org/wiki/Disk_encryption_key_escrow_use_cases.

20.1. volume_key Commands

The format for volume_key is:
volume_key [OPTION]... OPERAND
The operands and mode of operation for volume_key are determined by specifying one of the following options:
--save
This command expects the operand volume [packet]. If a packet is provided then volume_key will extract the keys and passphrases from it. If packet is not provided, then volume_key will extract the keys and passphrases from the volume, prompting the user where necessary. These keys and passphrases will then be stored in one or more output packets.
--restore
This command expects the operands volume packet. It then opens the volume and uses the keys and passphrases in the packet to make the volume accessible again, prompting the user where necessary, such as allowing the user to enter a new passphrase, for example.
--setup-volume
This command expects the operands volume packet name. It then opens the volume and uses the keys and passphrases in the packet to set up the volume for use of the decrypted data as name.
Name is the name of a dm-crypt volume. This operation makes the decrypted volume available as /dev/mapper/name.
This operation does not permanently alter the volume by adding a new passphrase, for example. The user can access and modify the decrypted volume, modifying volume in the process.
--reencrypt, --secrets, and --dump
These three commands perform similar functions with varying output methods. They each require the operand packet, and each opens the packet, decrypting it where necessary. --reencrypt then stores the information in one or more new output packets. --secrets outputs the keys and passphrases contained in the packet. --dump outputs the content of the packet, though the keys and passphrases are not output by default. This can be changed by appending --with-secrets to the command. It is also possible to only dump the unencrypted parts of the packet, if any, by using the --unencrypted command. This does not require any passphrase or private key access.
Each of these can be appended with the following options:
-o, --output packet
This command writes the default key or passphrase to the packet. The default key or passphrase depends on the volume format. Ensure it is one that is unlikely to expire, and will allow --restore to restore access to the volume.
--output-format format
This command uses the specified format for all output packets. Currently, format can be one of the following:
  • asymmetric: uses CMS to encrypt the whole packet, and requires a certificate
  • asymmetric_wrap_secret_only: wraps only the secret, or keys and passphrases, and requires a certificate
  • passphrase: uses GPG to encrypt the whole packet, and requires a passphrase
--create-random-passphrase packet
This command generates a random alphanumeric passphrase, adds it to the volume (without affecting other passphrases), and then stores this random passphrase into the packet.

20.2. Using volume_key as an Individual User

As an individual user, volume_key can be used to save encryption keys by using the following procedure.

Note

For all examples in this file, /path/to/volume is a LUKS device, not the plaintext device contained within. blkid -s type /path/to/volume should report type="crypto_LUKS".

Procedure 20.1. Using volume_key Stand-alone

  1. Run:
    volume_key --save /path/to/volume -o escrow-packet
    A prompt will then appear requiring an escrow packet passphrase to protect the key.
  2. Save the generated escrow-packet file, ensuring that the passphrase is not forgotten.
If the volume passphrase is forgotten, use the saved escrow packet to restore access to the data.

Procedure 20.2. Restore Access to Data with Escrow Packet

  1. Boot the system in an environment where volume_key can be run and the escrow packet is available (a rescue mode, for example).
  2. Run:
    volume_key --restore /path/to/volume escrow-packet
    A prompt will appear for the escrow packet passphrase that was used when creating the escrow packet, and for the new passphrase for the volume.
  3. Mount the volume using the chosen passphrase.
To free up the passphrase slot in the LUKS header of the encrypted volume, remove the old, forgotten passphrase by using the command cryptsetup luksKillSlot.

20.3. Using volume_key in a Larger Organization

In a larger organization, using a single password known by every system administrator and keeping track of a separate password for each system is impractical and a security risk. To counter this, volume_key can use asymmetric cryptography to minimize the number of people who know the password required to access encrypted data on any computer.
This section will cover the procedures required for preparation before saving encryption keys, how to save encryption keys, restoring access to a volume, and setting up emergency passphrases.

20.3.1. Preparation for Saving Encryption Keys

In order to begin saving encryption keys, some preparation is required.

Procedure 20.3. Preparation

  1. Create an X509 certificate/private pair.
  2. Designate trusted users who are trusted not to compromise the private key. These users will be able to decrypt the escrow packets.
  3. Choose which systems will be used to decrypt the escrow packets. On these systems, set up an NSS database that contains the private key.
    If the private key was not created in an NSS database, follow these steps:
    • Store the certificate and private key in an PKCS#12 file.
    • Run:
      certutil -d /the/nss/directory -N
      At this point it is possible to choose an NSS database password. Each NSS database can have a different password so the designated users do not need to share a single password if a separate NSS database is used by each user.
    • Run:
      pk12util -d /the/nss/directory -i the-pkcs12-file
  4. Distribute the certificate to anyone installing systems or saving keys on existing systems.
  5. For saved private keys, prepare storage that allows them to be looked up by machine and volume. For example, this can be a simple directory with one subdirectory per machine, or a database used for other system management tasks as well.

20.3.2. Saving Encryption Keys

After completing the required preparation (see Section 20.3.1, “Preparation for Saving Encryption Keys”) it is now possible to save the encryption keys using the following procedure.

Note

For all examples in this file, /path/to/volume is a LUKS device, not the plaintext device contained within; blkid -s type /path/to/volume should report type="crypto_LUKS".

Procedure 20.4. Saving Encryption Keys

  1. Run:
    volume_key --save /path/to/volume -c /path/to/cert escrow-packet
  2. Save the generated escrow-packet file in the prepared storage, associating it with the system and the volume.
These steps can be performed manually, or scripted as part of system installation.

20.3.3. Restoring Access to a Volume

After the encryption keys have been saved (see Section 20.3.1, “Preparation for Saving Encryption Keys” and Section 20.3.2, “Saving Encryption Keys”), access can be restored to a driver where needed.

Procedure 20.5. Restoring Access to a Volume

  1. Get the escrow packet for the volume from the packet storage and send it to one of the designated users for decryption.
  2. The designated user runs:
    volume_key --reencrypt -d /the/nss/directory escrow-packet-in -o escrow-packet-out
    After providing the NSS database password, the designated user chooses a passphrase for encrypting escrow-packet-out. This passphrase can be different every time and only protects the encryption keys while they are moved from the designated user to the target system.
  3. Obtain the escrow-packet-out file and the passphrase from the designated user.
  4. Boot the target system in an environment that can run volume_key and have the escrow-packet-out file available, such as in a rescue mode.
  5. Run:
    volume_key --restore /path/to/volume escrow-packet-out
    A prompt will appear for the packet passphrase chosen by the designated user, and for a new passphrase for the volume.
  6. Mount the volume using the chosen volume passphrase.
It is possible to remove the old passphrase that was forgotten by using cryptsetup luksKillSlot, for example, to free up the passphrase slot in the LUKS header of the encrypted volume. This is done with the command cryptsetup luksKillSlot device key-slot. For more information and examples see cryptsetup --help.

20.3.4. Setting up Emergency Passphrases

In some circumstances (such as traveling for business) it is impractical for system administrators to work directly with the affected systems, but users still need access to their data. In this case, volume_key can work with passphrases as well as encryption keys.
During the system installation, run:
volume_key --save /path/to/volume -c /path/to/ert --create-random-passphrase passphrase-packet
This generates a random passphrase, adds it to the specified volume, and stores it to passphrase-packet. It is also possible to combine the --create-random-passphrase and -o options to generate both packets at the same time.
If a user forgets the password, the designated user runs:
volume_key --secrets -d /your/nss/directory passphrase-packet
This shows the random passphrase. Give this passphrase to the end user.

20.4. volume_key References

More information on volume_key can be found:

Chapter 21. Solid-State Disk Deployment Guidelines

Solid-state disks (SSD) are storage devices that use NAND flash chips to persistently store data. This sets them apart from previous generations of disks, which store data in rotating, magnetic platters. In an SSD, the access time for data across the full Logical Block Address (LBA) range is constant; whereas with older disks that use rotating media, access patterns that span large address ranges incur seek costs. As such, SSD devices have better latency and throughput.
Performance degrades as the number of used blocks approaches the disk capacity. The degree of performance impact varies greatly by vendor. However, all devices experience some degradation.
To address the degradation issue, the host system (for example, the Linux kernel) may use discard requests to inform the storage that a given range of blocks is no longer in use. An SSD can use this information to free up space internally, using the free blocks for wear-leveling. Discards will only be issued if the storage advertises support in terms of its storage protocol (be it ATA or SCSI). Discard requests are issued to the storage using the negotiated discard command specific to the storage protocol (TRIM command for ATA, and WRITE SAME with UNMAP set, or UNMAP command for SCSI).
Enabling discard support is most useful when the following points are true:
  • Free space is still available on the file system.
  • Most logical blocks on the underlying storage device have already been written to.
For more information about UNMAP, see the section 4.7.3.4 of the SCSI Block Commands 3 T10 Specification.

Note

Not all solid-state devices in the market have discard support. To determine if your solid-state device has discard support, check for /sys/block/sda/queue/discard_granularity, which is the size of internal allocation unit of device.

Deployment Considerations

Because of the internal layout and operation of SSDs, it is best to partition devices on an internal erase block boundary. Partitioning utilities in Red Hat Enterprise Linux 7 chooses sane defaults if the SSD exports topology information. However, if the device does not export topology information, Red Hat recommends that the first partition should be created at a 1MB boundary.
SSD has various types of TRIM mechanism depending on the vendors choice. The early versions of disks improved the performance by compromising possible data leakage after the read command.
Following are the types of TRIM mechanism:
  • Non-deterministic TRIM
  • Deterministic TRIM (DRAT)
  • Deterministic Read Zero after TRIM (RZAT)
The first two types of TRIM mechanism can cause data leakage as the read command to the LBA after a TRIM returns different or same data. RZAT returns zero after the read command and Red Hat recommends this TRIM mechanism to avoid data leakage. It is affected only in SSD. Choose the disk which supports RZAT mechanism.
Type of TRIM mechanism used depends on hardware implementation. To find the type of TRIM mechanism on ATA, use the hdparm command. See the following example to find the type of TRIM mechanism:
# hdparm -I /dev/sda | grep TRIM
Data Set Management TRIM supported (limit 8 block)
Deterministic read data after TRIM
For more information, see man hdparm.
The Logical Volume Manager (LVM), the device-mapper (DM) targets, and MD (software raid) targets that LVM uses support discards. The only DM targets that do not support discards are dm-snapshot, dm-crypt, and dm-raid45. Discard support for the dm-mirror was added in Red Hat Enterprise Linux 6.1 and as of 7.0 MD supports discards.
Using RAID level 5 over SSD results in low performance if SSDs do not handle discard correctly. You can set discard in the raid456.conf file, or in the GRUB2 configuration. For instructions, see the following procedures.

Procedure 21.1. Setting discard in raid456.conf

The devices_handle_discard_safely module parameter is set in the raid456 module. To enable discard in the raid456.conf file:
  1. Verify that your hardware supports discards:
    # cat /sys/block/disk-name/queue/discard_zeroes_data
    If the returned value is 1, discards are supported. If the command returns 0, the RAID code has to zero the disk out, which takes more time.
  2. Create the /etc/modprobe.d/raid456.conf file, and include the following line:
    options raid456 devices_handle_discard_safely=Y
    
  3. Use the dracut -f command to rebuild the initial ramdisk (initrd).
  4. Reboot the system for the changes to take effect.

Procedure 21.2. Setting discard in the GRUB2 Configuration

The devices_handle_discard_safely module parameter is set in the raid456 module. To enable discard in the GRUB2 configuration:
  1. Verify that your hardware supports discards:
    # cat /sys/block/disk-name/queue/discard_zeroes_data
    If the returned value is 1, discards are supported. If the command returns 0, the RAID code has to zero the disk out, which takes more time.
  2. Add the following line to the /etc/default/grub file:
    raid456.devices_handle_discard_safely=Y
    
  3. The location of the GRUB2 configuration file is different on systems with the BIOS firmware and on systems with UEFI. Use one of the following commands to recreate the GRUB2 configuration file.
    • On a system with the BIOS firmware, use:
      # grub2-mkconfig -o /boot/grub2/grub.cfg
    • On a system with the UEFI firmware, use:
      # grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
  4. Reboot the system for the changes to take effect.

Note

In Red Hat Enterprise Linux 7, discard is fully supported by the ext4 and XFS file systems only.
In Red Hat Enterprise Linux 6.3 and earlier, only the ext4 file system fully supports discard. Starting with Red Hat Enterprise Linux 6.4, both ext4 and XFS file systems fully support discard. To enable discard commands on a device, use the discard option of the mount command. For example, to mount /dev/sda2 to /mnt with discard enabled, use:
# mount -t ext4 -o discard /dev/sda2 /mnt
By default, ext4 does not issue the discard command to, primarily, avoid problems on devices which might not properly implement discard. The Linux swap code issues discard commands to discard-enabled devices, and there is no option to control this behavior.

Performance Tuning Considerations

For information on performance tuning considerations regarding solid-state disks, see the Solid-State Disks section in the Red Hat Enterprise Linux 7 Performance Tuning Guide.

Chapter 22. Write Barriers

A write barrier is a kernel mechanism used to ensure that file system metadata is correctly written and ordered on persistent storage, even when storage devices with volatile write caches lose power. File systems with write barriers enabled ensures that data transmitted via fsync() is persistent throughout a power loss.
Enabling write barriers incurs a substantial performance penalty for some applications. Specifically, applications that use fsync() heavily or create and delete many small files will likely run much slower.

22.1. Importance of Write Barriers

File systems safely update metadata, ensuring consistency. Journalled file systems bundle metadata updates into transactions and send them to persistent storage in the following manner:
  1. The file system sends the body of the transaction to the storage device.
  2. The file system sends a commit block.
  3. If the transaction and its corresponding commit block are written to disk, the file system assumes that the transaction will survive any power failure.
However, file system integrity during power failure becomes more complex for storage devices with extra caches. Storage target devices like local S-ATA or SAS drives may have write caches ranging from 32MB to 64MB in size (with modern drives). Hardware RAID controllers often contain internal write caches. Further, high end arrays, like those from NetApp, IBM, Hitachi and EMC (among others), also have large caches.
Storage devices with write caches report I/O as "complete" when the data is in cache; if the cache loses power, it loses its data as well. Worse, as the cache de-stages to persistent storage, it may change the original metadata ordering. When this occurs, the commit block may be present on disk without having the complete, associated transaction in place. As a result, the journal may replay these uninitialized transaction blocks into the file system during post-power-loss recovery; this will cause data inconsistency and corruption.

How Write Barriers Work

Write barriers are implemented in the Linux kernel via storage write cache flushes before and after the I/O, which is order-critical. After the transaction is written, the storage cache is flushed, the commit block is written, and the cache is flushed again. This ensures that:
  • The disk contains all the data.
  • No re-ordering has occurred.
With barriers enabled, an fsync() call also issues a storage cache flush. This guarantees that file data is persistent on disk even if power loss occurs shortly after fsync() returns.

22.2. Enabling and Disabling Write Barriers

To mitigate the risk of data corruption during power loss, some storage devices use battery-backed write caches. Generally, high-end arrays and some hardware controllers use battery-backed write caches. However, because the cache's volatility is not visible to the kernel, Red Hat Enterprise Linux 7 enables write barriers by default on all supported journaling file systems.

Note

Write caches are designed to increase I/O performance. However, enabling write barriers means constantly flushing these caches, which can significantly reduce performance.
For devices with non-volatile, battery-backed write caches and those with write-caching disabled, you can safely disable write barriers at mount time using the -o nobarrier option for mount. However, some devices do not support write barriers; such devices log an error message to /var/log/messages. For more information, see Table 22.1, “Write Barrier Error Messages per File System”.
Table 22.1. Write Barrier Error Messages per File System
File SystemError Message
ext3/ext4JBD: barrier-based sync failed on device - disabling barriers
XFSFilesystem device - Disabling barriers, trial barrier write failed
btrfsbtrfs: disabling barriers on dev device

22.3. Write Barrier Considerations

Some system configurations do not need write barriers to protect data. In most cases, other methods are preferable to write barriers, since enabling write barriers causes a significant performance penalty.

Disabling Write Caches

One way to alternatively avoid data integrity issues is to ensure that no write caches lose data on power failures. When possible, the best way to configure this is to disable the write cache. On a simple server or desktop with one or more SATA drives (off a local SATA controller Intel AHCI part), you can disable the write cache on the target SATA drives with the following command:
# hdparm -W0 /device/

Battery-Backed Write Caches

Write barriers are also unnecessary whenever the system uses hardware RAID controllers with battery-backed write cache. If the system is equipped with such controllers and if its component drives have write caches disabled, the controller acts as a write-through cache; this informs the kernel that the write cache data survives a power loss.
Most controllers use vendor-specific tools to query and manipulate target drives. For example, the LSI Megaraid SAS controller uses a battery-backed write cache; this type of controller requires the MegaCli64 tool to manage target drives. To show the state of all back-end drives for LSI Megaraid SAS, use:
# MegaCli64 -LDGetProp -DskCache -LAll -aALL
To disable the write cache of all back-end drives for LSI Megaraid SAS, use:
# MegaCli64 -LDSetProp -DisDskCache -Lall -aALL

Note

Hardware RAID cards recharge their batteries while the system is operational. If a system is powered off for an extended period of time, the batteries will lose their charge, leaving stored data vulnerable during a power failure.

High-End Arrays

High-end arrays have various ways of protecting data in the event of a power failure. As such, there is no need to verify the state of the internal drives in external RAID storage.

NFS

NFS clients do not need to enable write barriers, since data integrity is handled by the NFS server side. As such, NFS servers should be configured to ensure data persistence throughout a power loss (whether through write barriers or other means).

Chapter 23. Storage I/O Alignment and Size

Recent enhancements to the SCSI and ATA standards allow storage devices to indicate their preferred (and in some cases, required) I/O alignment and I/O size. This information is particularly useful with newer disk drives that increase the physical sector size from 512 bytes to 4k bytes. This information may also be beneficial for RAID devices, where the chunk size and stripe size may impact performance.
The Linux I/O stack has been enhanced to process vendor-provided I/O alignment and I/O size information, allowing storage management tools (parted, lvm, mkfs.*, and the like) to optimize data placement and access. If a legacy device does not export I/O alignment and size data, then storage management tools in Red Hat Enterprise Linux 7 will conservatively align I/O on a 4k (or larger power of 2) boundary. This will ensure that 4k-sector devices operate correctly even if they do not indicate any required/preferred I/O alignment and size.
For information on determining the information that the operating system obtained from the device, see the Section 23.2, “Userspace Access”. This data is subsequently used by the storage management tools to determine data placement.
The IO scheduler has changed for Red Hat Enterprise Linux 7. Default IO Scheduler is now Deadline, except for SATA drives. CFQ is the default IO scheduler for SATA drives. For faster storage, Deadline outperforms CFQ and when it is used there is a performance increase without the need of special tuning.
If default is not right for some disks (for example, SAS rotational disks), then change the IO scheduler to CFQ. This instance will depend on the workload.

23.1. Parameters for Storage Access

The operating system uses the following information to determine I/O alignment and size:
physical_block_size
Smallest internal unit on which the device can operate
logical_block_size
Used externally to address a location on the device
alignment_offset
The number of bytes that the beginning of the Linux block device (partition/MD/LVM device) is offset from the underlying physical alignment
minimum_io_size
The device’s preferred minimum unit for random I/O
optimal_io_size
The device’s preferred unit for streaming I/O
For example, certain 4K sector devices may use a 4K physical_block_size internally but expose a more granular 512-byte logical_block_size to Linux. This discrepancy introduces potential for misaligned I/O. To address this, the Red Hat Enterprise Linux 7 I/O stack will attempt to start all data areas on a naturally-aligned boundary (physical_block_size) by making sure it accounts for any alignment_offset if the beginning of the block device is offset from the underlying physical alignment.
Storage vendors can also supply I/O hints about the preferred minimum unit for random I/O (minimum_io_size) and streaming I/O (optimal_io_size) of a device. For example, minimum_io_size and optimal_io_size may correspond to a RAID device's chunk size and stripe size respectively.

23.2. Userspace Access

Always take care to use properly aligned and sized I/O. This is especially important for Direct I/O access. Direct I/O should be aligned on a logical_block_size boundary, and in multiples of the logical_block_size.
With native 4K devices (i.e. logical_block_size is 4K) it is now critical that applications perform direct I/O in multiples of the device's logical_block_size. This means that applications will fail with native 4k devices that perform 512-byte aligned I/O rather than 4k-aligned I/O.
To avoid this, an application should consult the I/O parameters of a device to ensure it is using the proper I/O alignment and size. As mentioned earlier, I/O parameters are exposed through the both sysfs and block device ioctl interfaces.
For more information, see man libblkid. This man page is provided by the libblkid-devel package.

sysfs Interface

  • /sys/block/disk/alignment_offset
    or
    /sys/block/disk/partition/alignment_offset

    Note

    The file location depends on whether the disk is a physical disk (be that a local disk, local RAID, or a multipath LUN) or a virtual disk. The first file location is applicable to physical disks while the second file location is applicable to virtual disks. The reason for this is because virtio-blk will always report an alignment value for the partition. Physical disks may or may not report an alignment value.
  • /sys/block/disk/queue/physical_block_size
  • /sys/block/disk/queue/logical_block_size
  • /sys/block/disk/queue/minimum_io_size
  • /sys/block/disk/queue/optimal_io_size
The kernel will still export these sysfs attributes for "legacy" devices that do not provide I/O parameters information, for example:

Example 23.1. sysfs Interface

alignment_offset:    0
physical_block_size: 512
logical_block_size:  512
minimum_io_size:     512
optimal_io_size:     0

Block Device ioctls

  • BLKALIGNOFF: alignment_offset
  • BLKPBSZGET: physical_block_size
  • BLKSSZGET: logical_block_size
  • BLKIOMIN: minimum_io_size
  • BLKIOOPT: optimal_io_size

23.3. I/O Standards

This section describes I/O standards used by ATA and SCSI devices.

ATA

ATA devices must report appropriate information via the IDENTIFY DEVICE command. ATA devices only report I/O parameters for physical_block_size, logical_block_size, and alignment_offset. The additional I/O hints are outside the scope of the ATA Command Set.

SCSI

I/O parameters support in Red Hat Enterprise Linux 7 requires at least version 3 of the SCSI Primary Commands (SPC-3) protocol. The kernel will only send an extended inquiry (which gains access to the BLOCK LIMITS VPD page) and READ CAPACITY(16) command to devices which claim compliance with SPC-3.
The READ CAPACITY(16) command provides the block sizes and alignment offset:
  • LOGICAL BLOCK LENGTH IN BYTES is used to derive /sys/block/disk/queue/physical_block_size
  • LOGICAL BLOCKS PER PHYSICAL BLOCK EXPONENT is used to derive /sys/block/disk/queue/logical_block_size