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Configuring and managing logical volumes

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Red Hat Enterprise Linux 8

Configuring and managing LVM

Red Hat Customer Content Services

Abstract

Logical Volume Manager (LVM) is a storage virtualization software designed to enhance the management and flexibility of physical storage devices. By abstracting the physical hardware, LVM allows you to dynamically create, resize, and remove of virtual storage devices. Within this framework, physical volumes (PVs) represent the raw storage devices that are grouped together to form a volume group (VG). Within this VG, LVM allocates space to create a logical volume (LV). An LV is a virtual block storage device that a file system, database, or application can use.

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Chapter 1. Overview of logical volume management

Logical Volume Manager (LVM) creates a layer of abstraction over physical storage, which helps you to create logical storage volumes. This offers more flexibility compared to direct physical storage usage.

In addition, the hardware storage configuration is hidden from the software so you can resize and move it without stopping applications or unmounting file systems. This can reduce operational costs.

1.1. LVM architecture

The following are the components of LVM:

Physical volume
A physical volume (PV) is a partition or whole disk designated for LVM use. For more information, see Managing LVM physical volumes.
Volume group
A volume group (VG) is a collection of physical volumes (PVs), which creates a pool of disk space out of which you can allocate logical volumes. For more information, see Managing LVM volume groups.
Logical volume
A logical volume represents a usable storage device. For more information, see Managing LVM logical volumes.

The following diagram illustrates the components of LVM:

Figure 1.1. LVM logical volume components

LVM Logical Volume Components

1.2. Advantages of LVM

Logical volumes provide the following advantages over using physical storage directly:

Flexible capacity
When using logical volumes, you can aggregate devices and partitions into a single logical volume. With this functionality, file systems can extend across multiple devices as though they were a single, large one.
Convenient device naming
Logical storage volumes can be managed with user-defined and custom names.
Resizeable storage volumes
You can extend logical volumes or reduce logical volumes in size with simple software commands, without reformatting and repartitioning the underlying devices. For more information, see Modifying the size of a logical volume.
Online data relocation

To deploy newer, faster, or more resilient storage subsystems, you can move data while your system is active using the pvmove command. Data can be rearranged on disks while the disks are in use. For example, you can empty a hot-swappable disk before removing it.

For more information on how to migrate the data, see the pvmove man page and Removing physical volumes from a volume group.

Striped Volumes
You can create a logical volume that stripes data across two or more devices. This can dramatically increase throughput. For more information, see Extending a striped logical volume.
RAID volumes
Logical volumes provide a convenient way to configure RAID for your data. This provides protection against device failure and improves performance. For more information, see Configuring RAID logical volumes.
Volume snapshots
You can take snapshots, which is a point-in-time copy of logical volumes for consistent backups or to test the effect of changes without affecting the real data. For more information, see Snapshot of logical volumes.
Thin volumes
Logical volumes can be thin-provisioned. This allows you to create logical volumes that are larger than the available physical space. For more information, see Creating and managing thin provisioned volumes (thin volumes).
Caching
Caching uses fast devices, like SSDs, to cache data from logical volumes, boosting performance. For more information, see Enabling caching to improve logical volume performance.

Additional resources

Chapter 2. Managing LVM physical volumes

A physical volume (PV) is a physical storage device or a partition on a storage device that LVM uses.

During the initialization process, an LVM disk label and metadata are written to the device, which allows LVM to track and manage it as part of the logical volume management scheme.

Note

You cannot increase the size of the metadata after the initialization. If you need larger metadata, you must set the appropriate size during the initialization process.

When initialization process is complete, you can allocate the PV to a volume group (VG). You can divide this VG into logical volumes (LVs), which are the virtual block devices that operating systems and applications can use for storage.

To ensure optimal performance, partition the whole disk as a single PV for LVM use.

2.1. Creating an LVM physical volume

You can use the pvcreate command to initialize a physical volume LVM usage.

Prerequisites

  • Administrative access.
  • The lvm2 package is installed.

Procedure

  1. Identify the storage device you want to use as a physical volume. To list all available storage devices, use:

    $ lsblk
  2. Create an LVM physical volume:

    # pvcreate /dev/sdb

    Replace /dev/sdb with the name of the device you want to initialize as a physical volume.

Verification steps

  • Display the created physical volume:

    # pvs
    
      PV         VG  Fmt  Attr PSize  PFree
      /dev/sdb       lvm2 a--  28.87g 13.87g

Additional resources

  • pvcreate(8), pvdisplay(8), pvs(8), pvscan(8), and lvm(8) man pages on your system

2.2. Removing LVM physical volumes

You can use the pvremove command to remove a physical volume for LVM usage.

Prerequisites

  • Administrative access.

Procedure

  1. List the physical volumes to identify the device you want to remove:

    # pvs
    
      PV           VG Fmt  Attr PSize  PFree
      /dev/sdb1       lvm2 ---  28.87g 28.87g
  2. Remove the physical volume:

    # pvremove /dev/sdb1

    Replace /dev/sdb1 with the name of the device associated with the physical volume.

Note

If your physical volume is part of the volume group, you need to remove it from the volume group first.

  • If you volume group contains more that one physical volume, use the vgreduce command:

    # vgreduce VolumeGroupName /dev/sdb1

    Replace VolumeGroupName with the name of the volume group. Replace /dev/sdb1 with the name of the device.

  • If your volume group contains only one physical volume, use vgremove command:

    # vgremove VolumeGroupName

    Replace VolumeGroupName with the name of the volume group.

Verification

  • Verify the physical volume is removed:

    # pvs

Additional resources

  • pvremove(8) man page on your system

2.3. Creating logical volumes in the web console

Logical volumes act as physical drives. You can use the RHEL 8 web console to create LVM logical volumes in a volume group.

Prerequisites

  • You have installed the RHEL 8 web console.

    For instructions, see Installing and enabling the web console.

  • The cockpit-storaged package is installed on your system.
  • The volume group is created.

Procedure

  1. Log in to the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Storage.
  3. In the Storage table, click the volume group in which you want to create logical volumes.
  4. On the Logical volume group page, scroll to the LVM2 logical volumes section and click Create new logical volume.
  5. In the Name field, enter a name for the new logical volume. Do not include spaces in the name.
  6. In the Purpose drop-down menu, select Block device for filesystems.

    This configuration enables you to create a logical volume with the maximum volume size which is equal to the sum of the capacities of all drives included in the volume group.

    cockpit lv block dev

  7. Define the size of the logical volume. Consider:

    • How much space the system using this logical volume will need.
    • How many logical volumes you want to create.

    You do not have to use the whole space. If necessary, you can grow the logical volume later.

    cockpit lv size

  8. Click Create.

    The logical volume is created. To use the logical volume you must format and mount the volume.

Verification

  • On the Logical volume page, scroll to the LVM2 logical volumes section and verify whether the new logical volume is listed.

    cockpit lv details

2.4. Formatting logical volumes in the web console

Logical volumes act as physical drives. To use them, you must format them with a file system.

Warning

Formatting logical volumes erases all data on the volume.

The file system you select determines the configuration parameters you can use for logical volumes. For example, the XFS file system does not support shrinking volumes.

Prerequisites

  • You have installed the RHEL 8 web console.

    For instructions, see Installing and enabling the web console.

  • The cockpit-storaged package is installed on your system.
  • The logical volume created.
  • You have root access privileges to the system.

Procedure

  1. Log in to the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Storage.
  3. In the Storage table, click the volume group in the logical volumes is created.
  4. On the Logical volume group page, scroll to the LVM2 logical volumes section.
  5. Click the menu button, , next to the volume group you want to format.
  6. From the drop-down menu, select Format.

    Image displaying the details of the existing logical volume.

  7. In the Name field, enter a name for the file system.
  8. In the Mount Point field, add the mount path.

    The format a logical volume dialog box with configurable fields.

  9. In the Type drop-down menu, select a file system:

    • XFS file system supports large logical volumes, switching physical drives online without outage, and growing an existing file system. Leave this file system selected if you do not have a different strong preference.

      XFS does not support reducing the size of a volume formatted with an XFS file system

    • ext4 file system supports:

      • Logical volumes
      • Switching physical drives online without an outage
      • Growing a file system
      • Shrinking a file system
  10. Select the Overwrite existing data with zeros checkbox if you want the RHEL web console to rewrite the whole disk with zeros. This option is slower because the program has to go through the whole disk, but it is more secure. Use this option if the disk includes any data and you need to overwrite it.

    If you do not select the Overwrite existing data with zeros checkbox, the RHEL web console rewrites only the disk header. This increases the speed of formatting.

  11. From the Encryption drop-down menu, select the type of encryption if you want to enable it on the logical volume.

    You can select a version with either the LUKS1 (Linux Unified Key Setup) or LUKS2 encryption, which allows you to encrypt the volume with a passphrase.

  12. In the At boot drop-down menu, select when you want the logical volume to mount after the system boots.
  13. Select the required Mount options.
  14. Format the logical volume:

    • If you want to format the volume and immediately mount it, click Format and mount.
    • If you want to format the volume without mounting it, click Format only.

      Formatting can take several minutes depending on the volume size and which formatting options are selected.

Verification

  1. On the Logical volume group page, scroll to the LVM2 logical volumes section and click the logical volume to check the details and additional options.

    cockpit lv formatted

  2. If you selected the Format only option, click the menu button at the end of the line of the logical volume, and select Mount to use the logical volume.

2.5. Resizing logical volumes in the web console

You can extend or reduce logical volumes in the RHEL 8 web console. The example procedure demonstrates how to grow and shrink the size of a logical volume without taking the volume offline.

Warning

You cannot reduce volumes that contains GFS2 or XFS filesystem.

Prerequisites

  • You have installed the RHEL 8 web console.

    For instructions, see Installing and enabling the web console.

  • The cockpit-storaged package is installed on your system.
  • An existing logical volume containing a file system that supports resizing logical volumes.

Procedure

  1. Log in to the RHEL web console.
  2. Click Storage.
  3. In the Storage table, click the volume group in the logical volumes is created.
  4. On the Logical volume group page, scroll to the LVM2 logical volumes section and click the menu button, , next to volume group you want to resize.

    LVM2 volume group details

  5. From the menu, select Grow or Shrink to resize the volume:

    • Growing the Volume:

      1. Select Grow to increase the size of the volume.
      2. In the Grow logical volume dialog box, adjust the size of the logical volume.

        LV grow dialog

      3. Click Grow.

        LVM grows the logical volume without causing a system outage.

    • Shrinking the Volume:

      1. Select Shrink to reduce the size of the volume.
      2. In the Shrink logical volume dialog box, adjust the size of the logical volume.

        LV shrink dialog

      3. Click Shrink.

        LVM shrinks the logical volume without causing a system outage.

2.6. Additional resources

Chapter 3. Managing LVM volume groups

You can create and use volume groups (VGs) to manage and resize multiple physical volumes (PVs) combined into a single storage entity.

Extents are the smallest units of space that you can allocate in LVM. Physical extents (PE) and logical extents (LE) has the default size of 4 MiB that you can configure. All extents have the same size.

When you create a logical volume (LV) within a VG, LVM allocates physical extents on the PVs. The logical extents within the LV correspond one-to-one with physical extents in the VG. You do not need to specify the PEs to create LVs. LVM will locate the available PEs and piece them together to create a LV of the requested size.

Within a VG, you can create multiple LVs, each acting like a traditional partition but with the ability to span across physical volumes and resize dynamically. VGs can manage the allocation of disk space automatically.

3.1. Creating an LVM volume group

You can use the vgcreate command to create a volume group (VG). You can adjust the extent size for very large or very small volumes to optimize performance and storage efficiency. You can specify the extent size when creating a VG. To change the extent size you must re-create the volume group.

Prerequisites

  • Administrative access.
  • The lvm2 package is installed.
  • One or more physical volumes are created. For more information about creating physical volumes, see Creating LVM physical volume.

Procedure

  1. List and identify the PV that you want to include in the VG:

    # pvs
  2. Create a VG:

    # vgcreate VolumeGroupName PhysicalVolumeName1 PhysicalVolumeName2

    Replace VolumeGroupName with the name of the volume group that you want to create. Replace PhysicalVolumeName with the name of the PV.

    To specify the extent size when creating a VG, use the -s ExtentSize option. Replace ExtentSize with the size of the extent. If you provide no size suffix, the command defaults to MB.

Verification

  • Verify that the VG is created:

    # vgs
    
      VG              #PV #LV #SN Attr   VSize  VFree
      VolumeGroupName   1   0   0 wz--n- 28.87g 28.87g

Additional resources

  • vgcreate(8), vgs(8), and pvs(8) man pages on your system

3.2. Creating volume groups in the web console

Create volume groups from one or more physical drives or other storage devices.

Logical volumes are created from volume groups. Each volume group can include multiple logical volumes.

Prerequisites

  • You have installed the RHEL 8 web console.

    For instructions, see Installing and enabling the web console.

  • The cockpit-storaged package is installed on your system.
  • Physical drives or other types of storage devices from which you want to create volume groups.

Procedure

  1. Log in to the RHEL 8 web console.

    For details, see Logging in to the web console.

  2. Click Storage.
  3. In the Storage table, click the menu button.
  4. From the drop-down menu, select Create LVM2 volume group.

    Image displaying the available options in the Storage table drop-down menu. Selecting Create LVM2 volume group.

  5. In the Name field, enter a name for the volume group. The name must not include spaces.
  6. Select the drives you want to combine to create the volume group.

    cockpit create volume group

    The RHEL web console displays only unused block devices. If you do not see your device in the list, make sure that it is not being used by your system, or format it to be empty and unused. Used devices include, for example:

    • Devices formatted with a file system
    • Physical volumes in another volume group
    • Physical volumes being a member of another software RAID device
  7. Click Create.

    The volume group is created.

Verification

  • On the Storage page, check whether the new volume group is listed in the Storage table.

3.3. Renaming an LVM volume group

You can use the vgrename command to rename a volume group (VG).

Prerequisites

Procedure

  1. List and identify the VG that you want to rename:

    # vgs
  2. Rename the VG:

    # vgrename OldVolumeGroupName NewVolumeGroupName

    Replace OldVolumeGroupName with the name of the VG. Replace NewVolumeGroupName with the new name for the VG.

Verification

  • Verify that the VG has a new name:

    # vgs
    
      VG                  #PV #LV #SN Attr   VSize  VFree
      NewVolumeGroupName   1   0   0 wz--n- 28.87g 28.87g

Additional resources

  • vgrename(8), vgs(8) man pages

3.4. Extending an LVM volume group

You can use the vgextend command to add physical volumes (PVs) to a volume group (VG).

Prerequisites

Procedure

  1. List and identify the VG that you want to extend:

    # vgs
  2. List and identify the PVs that you want to add to the VG:

    # pvs
  3. Extend the VG:

    # vgextend VolumeGroupName PhysicalVolumeName

    Replace VolumeGroupName with the name of the VG. Replace PhysicalVolumeName with the name of the PV.

Verification

  • Verify that the VG now includes the new PV:

    # pvs
    
      PV         VG              Fmt  Attr PSize  PFree
      /dev/sda   VolumeGroupName lvm2 a--  28.87g 28.87g
      /dev/sdd   VolumeGroupName lvm2 a--   1.88g  1.88g

Additional resources

  • vgextend(8), vgs(8), pvs(8) man pages

3.5. Combining LVM volume groups

You can combine two existing volume groups (VGs) with the vgmerge command. The source volume will be merged into the destination volume.

Prerequisites

Procedure

  1. List and identify the VG that you want to merge:

    # vgs
    
      VG               #PV #LV #SN Attr   VSize  VFree
      VolumeGroupName1   1   0   0 wz--n- 28.87g 28.87g
      VolumeGroupName2   1   0   0 wz--n-  1.88g  1.88g
  2. Merge the source VG into the destination VG:

    # vgmerge VolumeGroupName2 VolumeGroupName1

    Replace VolumeGroupName2 with the name of the source VG. Replace VolumeGroupName1 with the name of the destination VG.

Verification

  • Verify that the VG now includes the new PV:

    # vgs
    
      VG               #PV #LV #SN Attr   VSize  VFree
      VolumeGroupName1   2   0   0 wz--n- 30.75g 30.75g

Additional resources

  • vgmerge(8) man page on your system

3.6. Removing physical volumes from a volume group

To remove unused physical volumes (PVs) from a volume group (VG), use the vgreduce command. The vgreduce command shrinks a volume group’s capacity by removing one or more empty physical volumes. This frees those physical volumes to be used in different volume groups or to be removed from the system.

Procedure

  1. If the physical volume is still being used, migrate the data to another physical volume from the same volume group:

    # pvmove /dev/vdb3
      /dev/vdb3: Moved: 2.0%
     ...
      /dev/vdb3: Moved: 79.2%
     ...
      /dev/vdb3: Moved: 100.0%
  2. If there are not enough free extents on the other physical volumes in the existing volume group:

    1. Create a new physical volume from /dev/vdb4:

      # pvcreate /dev/vdb4
        Physical volume "/dev/vdb4" successfully created
    2. Add the newly created physical volume to the volume group:

      # vgextend VolumeGroupName /dev/vdb4
        Volume group "VolumeGroupName" successfully extended
    3. Move the data from /dev/vdb3 to /dev/vdb4:

      # pvmove /dev/vdb3 /dev/vdb4
        /dev/vdb3: Moved: 33.33%
        /dev/vdb3: Moved: 100.00%
  3. Remove the physical volume /dev/vdb3 from the volume group:

    # vgreduce VolumeGroupName /dev/vdb3
    Removed "/dev/vdb3" from volume group "VolumeGroupName"

Verification

  • Verify that the /dev/vdb3 physical volume is removed from the VolumeGroupName volume group:

    # pvs
      PV           VG                Fmt    Attr   PSize      PFree       Used
      /dev/vdb1 VolumeGroupName  lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb2 VolumeGroupName  lvm2   a--    1020.00m    0          1020.00m
      /dev/vdb3                    lvm2   a--    1020.00m   1008.00m    12.00m

Additional resources

  • vgreduce(8), pvmove(8), and pvs(8) man pages on your system

3.7. Splitting a LVM volume group

If there is enough unused space on the physical volumes, a new volume group can be created without adding new disks.

In the initial setup, the volume group VolumeGroupName1 consists of /dev/vdb1, /dev/vdb2, and /dev/vdb3. After completing this procedure, the volume group VolumeGroupName1 will consist of /dev/vdb1 and /dev/vdb2, and the second volume group, VolumeGroupName2, will consist of /dev/vdb3.

Prerequisites

  • You have sufficient space in the volume group. Use the vgscan command to determine how much free space is currently available in the volume group.
  • Depending on the free capacity in the existing physical volume, move all the used physical extents to other physical volume using the pvmove command. For more information, see Removing physical volumes from a volume group.

Procedure

  1. Split the existing volume group VolumeGroupName1 to the new volume group VolumeGroupName2:

    # vgsplit VolumeGroupName1 VolumeGroupName2 /dev/vdb3
      Volume group "VolumeGroupName2" successfully split from "VolumeGroupName1"
    Note

    If you have created a logical volume using the existing volume group, use the following command to deactivate the logical volume:

    # lvchange -a n /dev/VolumeGroupName1/LogicalVolumeName

    For more information about creating logical volumes, see Managing LVM logical volumes.

  2. View the attributes of the two volume groups:

    # vgs
      VG                  #PV #LV #SN Attr   VSize  VFree
      VolumeGroupName1     2   1   0 wz--n- 34.30G 10.80G
      VolumeGroupName2     1   0   0 wz--n- 17.15G 17.15G

Verification

  • Verify that the newly created volume group VolumeGroupName2 consists of /dev/vdb3 physical volume:

    # pvs
      PV          VG                  Fmt     Attr    PSize       PFree       Used
      /dev/vdb1 VolumeGroupName1   lvm2    a--     1020.00m      0        1020.00m
      /dev/vdb2 VolumeGroupName1   lvm2    a--     1020.00m      0        1020.00m
      /dev/vdb3 VolumeGroupName2   lvm2    a--     1020.00m    1008.00m    12.00m

Additional resources

  • vgsplit(8), vgs(8), and pvs(8) man pages on your system

3.8. Moving a volume group to another system

You can move an entire LVM volume group (VG) to another system using the following commands:

vgexport
Use this command on an existing system to make an inactive VG inaccessible to the system. Once the VG is inaccessible, you can detach its physical volumes (PV).
vgimport
Use this command on the other system to make the VG, which was inactive in the old system, accessible in the new system.

Prerequisites

  • No users are accessing files on the active volumes in the volume group that you are moving.

Procedure

  1. Unmount the LogicalVolumeName logical volume:

    # umount /dev/mnt/LogicalVolumeName
  2. Deactivate all logical volumes in the volume group, which prevents any further activity on the volume group:

    # vgchange -an VolumeGroupName
    vgchange -- volume group "VolumeGroupName" successfully deactivated
  3. Export the volume group to prevent it from being accessed by the system from which you are removing it:

    # vgexport VolumeGroupName
    vgexport -- volume group "VolumeGroupName" successfully exported
  4. View the exported volume group:

    # pvscan
      PV /dev/sda1    is in exported VG VolumeGroupName [17.15 GB / 7.15 GB free]
      PV /dev/sdc1    is in exported VG VolumeGroupName [17.15 GB / 15.15 GB free]
      PV /dev/sdd1    is in exported VG VolumeGroupName [17.15 GB / 15.15 GB free]
      ...
  5. Shut down your system and unplug the disks that make up the volume group and connect them to the new system.
  6. Plug the disks into the new system and import the volume group to make it accessible to the new system:

    # vgimport VolumeGroupName
    Note

    You can use the --force argument of the vgimport command to import volume groups that are missing physical volumes and subsequently run the vgreduce --removemissing command.

  7. Activate the volume group:

    # vgchange -ay VolumeGroupName
  8. Mount the file system to make it available for use:

    # mkdir -p /mnt/VolumeGroupName/users
    # mount /dev/VolumeGroupName/users /mnt/VolumeGroupName/users

Additional resources

  • vgimport(8), vgexport(8), and vgchange(8) man pages on your system

3.9. Removing LVM volume groups

You can remove an existing volume group using the vgremove command. Only volume groups that do not contain logical volumes can be removed.

Prerequisites

  • Administrative access.
  • The volume group contains no logical volumes. To remove logical volumes from a volume group, see Removing LVM logical volumes.

Procedure

  1. Ensure the volume group does not contain logical volumes:

    # vgs -o vg_name,lv_count VolumeGroupName
    
      VG               #LV
      VolumeGroupName    0

    Replace VolumeGroupName with the name of the volume group.

  2. Remove the volume group:

    # vgremove VolumeGroupName

    Replace VolumeGroupName with the name of the volume group.

Additional resources

  • vgs(8), vgremove(8) man pages on your system

3.10. Removing LVM volume groups in a cluster environment

In a cluster environment, LVM uses the lockspace <qualifier> to coordinate access to volume groups shared among multiple machines. You must stop the lockspace before removing a volume group to make sure no other node is trying to access or modify it during the removal process.

Prerequisites

  • Administrative access.
  • The volume group contains no logical volumes.

Procedure

  1. Ensure the volume group does not contain logical volumes:

    # vgs -o vg_name,lv_count VolumeGroupName
    
      VG               #LV
      VolumeGroupName    0

    Replace VolumeGroupName with the name of the volume group.

  2. Stop the lockspace on all nodes except the node where you are removing the volume group:

    # vgchange --lockstop VolumeGroupName

    Replace VolumeGroupName with the name of the volume group and wait for the lock to stop.

  3. Remove the volume group:

    # vgremove VolumeGroupName

    Replace VolumeGroupName with the name of the volume group.

Additional resources

  • vgremove(8), vgchange(8) man page

Chapter 4. Basic logical volume management

With LVM, you can do the following tasks:

  • Create new logical volumes to extend storage capabilities of your system
  • Extend existing volumes and thin pools to accommodate growing data
  • Rename volumes for better organization
  • Reduce volumes to free up unused space
  • Safely remove volumes when they are no longer needed
  • Activate or deactivate volumes to control the system’s access to its data

4.1. Overview of logical volume features

With the Logical Volume Manager (LVM), you can manage disk storage in a flexible and efficient way that traditional partitioning schemes cannot offer. Below is a summary of key LVM features that are used for storage management and optimization.

Concatenation
Concatenation involves combining space from one or more physical volumes into a singular logical volume, effectively merging the physical storage.
Striping
Striping optimizes data I/O efficiency by distributing data across multiple physical volumes. This method enhances performance for sequential reads and writes by allowing parallel I/O operations.
RAID
LVM supports RAID levels 0, 1, 4, 5, 6, and 10. When you create a RAID logical volume, LVM creates a metadata subvolume that is one extent in size for every data or parity subvolume in the array.
Thin provisioning
Thin provisioning enables the creation of logical volumes that are larger than the available physical storage. With thin provisioning, the system dynamically allocates storage based on actual usage instead of allocating a predetermined amount upfront.
Snapshots
With LVM snapshots, you can create point-in-time copies of logical volumes. A snapshot starts empty. As changes occur on the original logical volume, the snapshot captures the pre-change states through copy-on-write (CoW), growing only with changes to preserve the state of the original logical volume.
Caching
LVM supports the use of fast block devices, such as SSD drives as write-back or write-through caches for larger slower block devices. Users can create cache logical volumes to improve the performance of their existing logical volumes or create new cache logical volumes composed of a small and fast device coupled with a large and slow device.

4.2. Creating logical volumes

LVM provides a flexible approach to handling disk storage by abstracting the physical layer into logical volumes that can be created and adjusted based on your needs.

4.2.1. Creating a linear (thick) logical volume

With linear logical volumes (LVs), you can merge multiple physical storage units into one virtual storage space. You can easily expand or reduce linear LVs to accommodate the data requirements.

Prerequisites

  • Administrative access.
  • The lvm2 package is installed.
  • The volume group is created. For more information, see Creating LVM volume group.

Procedure

  1. List the names of volume groups and their size:

    # vgs -o vg_name,vg_size
    
      VG              VSize
      VolumeGroupName 30.75g
  2. Create a linear LV:

    # lvcreate --name LogicalVolumeName --size VolumeSize VolumeGroupName

    Replace LogicalVolumeName with the name of the LV. Replace VolumeSize with the size for the LV. If no size suffix is provided the command defaults to MB. Replace VolumeGroupName with the name of the volume group.

Verification

  • Verify that the linear LV is created:

    # lvs -o lv_name,seg_type
    
      LV                   Type
      LogicalVolumeName    linear

Additional resources

  • The vgs(8), lvs(8), lvcreate(8) man pages

4.2.2. Creating a striped logical volume

With striped logical volume (LV), you can distribute the data across multiple physical volumes (PVs), potentially increasing the read and write speed by utilizing the bandwidth of multiple disks simultaneously.

When creating a striped LV, it is important to consider the stripe number and size. The stripe number is the count of PVs across which data is distributed. Increasing the stripe number can enhance performance by utilizing multiple disks concurrently. Stripe size is the size of the data chunk written to each disk in the stripe set before moving to the next disk and is specified in kilobytes (KB). The optimal stripe size depends on your workload and the filesystem block size. The default is 64KB and can be adjusted.

Prerequisites

  • Administrative access.

Procedure

  1. List the names of volume groups and their size:

    # vgs -o vg_name,vg_size
    
      VG              VSize
      VolumeGroupName 30.75g
  2. Create a striped LV:

    # lvcreate --stripes NumberOfStripes --stripesize StripeSize --size LogicalVolumeSize --name LogicalVolumeName VolumeGroupName

    Replace NumberOfStripes with the number of stripes. Replace StripeSize with the stripe size in kilobytes. The --stripesize is not a required option. If you do not specify the stripe size it defaults to 64KB. Replace LogicalVolumeName with the name of the LV. Replace VolumeGroupName with the name of the volume group.

Verification

  • Verify that the striped LV is created:

    # lvs -o lv_name,seg_type
    
      LV                   Type
      LogicalVolumeName    striped

Additional resources

  • The vgs(8) lvs(8), lvcreate(8) man pages

4.2.3. Creating a RAID logical volume

RAID logical volumes enable you to use multiple disks for redundancy and performance. LVM supports various RAID levels, including RAID0, RAID1, RAID4, RAID5, RAID6, and RAID10.

With LVM you can create striped RAIDs (RAID0, RAID4, RAID5, RAID6), mirrored RAID (RAID1), or a combination of both (RAID10).

RAID 4, RAID 5, and RAID 6 offer fault tolerance by storing parity data that can be used to reconstruct lost information in case of a disk failure.

When creating RAID LVs, place each stripe on a separate PV. The number of stripes equals to the number of PVs that should be in the volume group (VG).

Table 4.1. Minimal RAID configuration requirements
RAID levelTypeParityMinimum number of devicesMinimum stripe number

RAID0

Striping

None

2

2

RAID1

Mirroring

None

2

-

RAID4

Striping

Uses first device to store parity

3

2

RAID5

Striping

Uses an extra device to store parity

3

2

RAID6

Striping

Uses two extra devices to store parity

5

3

RAID10

Striping and mirroring

None

4

2

Prerequisites

  • Administrative access.

Procedure

  1. List the names of volume groups and their size:

    # vgs -o vg_name,vg_size
    
      VG              VSize
      VolumeGroupName 30.75g
  2. Create a RAID LV:

    • To create a striped raid, use:

      # lvcreate --type raidlevel --stripes NumberOfStripes --stripesize StripeSize --size Size --name LogicalVolumeName VolumeGroupName

      Replace level with the RAID level 0, 4, 5, or 6. Replace NumberOfStripes with the number of stripes. Replace StripeSize with the stripe size in kilobytes. Replace Size with the size of the LV. Replace LogicalVolumeName with the name of the LV.

    • To create a mirrored RAID, use:

      # lvcreate --type raid1 --mirrors MirrorsNumber --size Size --name LogicalVolumeName VolumeGroupName

      Replace MirrorsNumber with the number of mirrors. Replace Size with the size of the LV. Replace LogicalVolumeName with the name of the LV.

    • To create a mirrored and striped RAID, use:

      # lvcreate --type raid10 --mirrors MirrorsNumber --stripes NumberOfStripes --stripesize StripeSize --size Size --name LogicalVolumeName VolumeGroupName

      Replace MirrorsNumber with the number of mirrors. Replace NumberOfStripes with the number of stripes. Replace StripeSize with the stripe size in kilobytes. Replace Size with the size of the LV. Replace LogicalVolumeName with the name of the LV.

Verification

  • Verify that the RAID LV is created:

    # lvs -o lv_name,seg_type
    
      LV                   Type
      LogicalVolumeName    raid0

Additional resources

  • lvmraid(7), vgs(8), lvs(8), lvcreate(8) man pages

4.2.4. Creating a thin logical volume

Under thin provisioning, physical extents (PEs) from a volume group (VG) are allocated to create a thin pool with a specific physical size. Logical volumes (LVs) are then allocated from this thin pool based on a virtual size, not limited by the pool’s physical capacity. With this, the virtual size of each thin LV can exceed the actual size of the thin pool leading to over-provisioning, when the collective virtual sizes of all thin LVs surpasses the physical capacity of the thin pool. Therefore, it is essential to monitor both logical and physical usage closely to avoid running out of space and outages.

Thin provisioning optimizes storage efficiency by allocating space as needed, lowering initial costs and improving resource utilization. However, when using thin LVs, beware of the following drawbacks:

  • Improper discard handling can block the release of unused storage space, causing full allocation of the space over time.
  • Copy on Write (CoW) operation can be slower on file systems with snapshots.
  • Data blocks can be intermixed between multiple file systems leading to random access limitations.

Prerequisites

Procedure

  1. List the names of volume groups and their size:

    # vgs -o vg_name,vg_size
    
      VG              VSize
      VolumeGroupName 30.75g
  2. Create a thin pool:

    # lvcreate --type thin-pool --size PoolSize --name ThinPoolName VolumeGroupName

    Replace PoolSize with the maximum amount of disk space the thin pool can use. Replace ThinPoolName with the name for the thin pool. Replace VolumeGroupName with the name of the volume group.

  3. Create a thin LV:

    # lvcreate --type thin --virtualsize MaxVolumeSize --name ThinVolumeName --thinpool ThinPoolName VolumeGroupName

    Replace MaxVolumeSize with the maximum size the volume can grow to within the thin pool. Replace ThinPoolName with the name for the thin pool. Replace VolumeGroupName with the name of the volume group.

    Note

    You can create other thin LVs within the same thin pool.

Verification

  • Verify that the thin LV is created:

    # lvs -o lv_name,seg_type
      LV                Type
      ThinPoolName      thin-pool
      ThinVolumeName    thin

Additional resources

  • The lvs(8), lvcreate(8) man pages

4.3. Resizing logical volumes

With Logical Volume Manager (LVM), you can resize logical volumes (LVs) as needed without affecting the data stored on them.

4.3.1. Extending a linear logical volume

You can extend linear (thick) LVs and their snapshots with the lvextend command.

Prerequisites

  • Administrative access.

Procedure

  1. Ensure your volume group has enough space to extend your LV:

    # lvs -o lv_name,lv_size,vg_name,vg_size,vg_free
      LV                   LSize   VG              VSize  VFree
      LogicalVolumeName    1.49g   VolumeGroupName 30.75g 29.11g
  2. Extend the linear LV and resize the file system:

    # lvextend --size +AdditionalSize --resizefs VolumeGroupName/LogicalVolumeName

    Replace AdditionalSize with how much space to add to the LV. The default unit of measurement is megabytes, but you can specify other units. Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the thin volume.

Verification

  • Verify that the linear LV is extended:

    # lvs -o lv_name,lv_size
      LV                   LSize
      NewLogicalVolumeName 6.49g

4.3.2. Extending a thin logical volume

You can extend the thin logical volume (LV) with the lvextend command.

Prerequisites

  • Administrative access.

Procedure

  1. Ensure the thin pool has enough space for the data you plan to add:

    # lvs -o lv_name,lv_size,data_percent
    
      LV                LSize   Data%
      MyThinPool        20.10g  3.21
      ThinVolumeName     1.10g  4.88
  2. Extend the thin LV and resize the file system:

    # lvextend --size +AdditionalSize --resizefs VolumeGroupName/ThinVolumeName

    Replace AdditionalSize with how much space to add to the LV. The default unit of measurement is megabytes, but you can specify other units. Replace VolumeGroupName with the name of the volume group. Replace ThinVolumeName with the name of the thin volume.

Verification

  • Verify the thin LV is extended:

    # lvs -o lv_name,lv_size,data_percent
    
      LV                LSize   Data%
      MyThinPool        20.10g  3.21
      ThinVolumeName     6.10g  0.43

4.3.3. Extending a thin pool

The virtual size of thin logical volumes can exceed the physical capacity of the thin pool resulting in over-provisioning. To prevent running out of space, you must monitor and periodically extend the capacity of the thin pool.

The data_percent metric indicates the percentage of the allocated data space that the thin pool currently uses. The metadata_percent metric reflects the percentage of space used for storing metadata, which is essential for managing the mappings within the thin pool.

Monitoring these metrics is vital to ensure efficient storage management and to avoid capacity issues.

LVM provides the option to manually extend the data or metadata capacity as needed. Alternatively, you can enable monitoring and automate the expansion of your thin pool.

4.3.3.1. Manually extending a thin pool

Logical Volume Manager (LVM) provides the option to manually extend the data segment, the metadata segment, or the thin pool.

4.3.3.1.1. Extending a thin pool

You can use the lvextend command to extend the thin pool.

Prerequisites

  • Administrative access.

Procedure

  1. Display the data and metadata space used:

    # lvs -o lv_name,seg_type,data_percent,metadata_percent
    
      LV                Type      Data%  Meta%
      ThinPoolName      thin-pool 97.66  26.86
      ThinVolumeName    thin      48.80
  2. Extend the thin pool:

    # lvextend -L Size VolumeGroupName/ThinPoolName

    Replace Size with the new size for your thin pool. Replace VolumeGroupName with the name of the volume group. Replace ThinPoolName with the name of the thin pool.

    The data size will be extended. The metadata size will be extended if necessary.

Verification

  • Verify that the thin pool is extended:

    # lvs -o lv_name,seg_type,data_percent,metadata_percent
    
      LV                Type      Data%  Meta%
      ThinPoolName      thin-pool 24.41  16.93
      ThinVolumeName    thin      24.41

Additional resources

  • The lvs(8), lvextend(8) man pages
  • lvs -o help
4.3.3.1.2. Extending a thin pool data segment

You can use the lvextend command to extend the data_percent segment.

Prerequisites

  • Administrative access.

Procedure

  1. Display the data_percent segment:

    # lvs -o lv_name,seg_type,data_percent
    
      LV                Type      Data%
      ThinPoolName      thin-pool 93.87
  2. Extend the data_percent segment:

    # lvextend -L Size VolumeGroupName/ThinPoolName_tdata

    Replace Size with the size for your data segment. Replace VolumeGroupName with name of the volume group. Replace ThinPoolName with the name of the thin pool.

Verification

  • Verify that the data_percent segment is extended:

    # lvs -o lv_name,seg_type,data_percent
    
      LV                Type      Data%
      ThinPoolName      thin-pool 40.23

Additional resources

  • The lvs(8), lvextend(8) man pages
  • lvs -o help
4.3.3.1.3. Extending a thin pool metadata segment

You can use the lvextend command to extend the metadata_percent segment.

Prerequisites

  • Administrative access.

Procedure

  1. Display the metadata_percent segment:

    # lvs -o lv_name,seg_type,metadata_percent
    
      LV                Type      Meta%
      ThinPoolName      thin-pool 75.00
  2. Extend the metadata_percent segment:

    # lvextend -L Size VolumeGroupName/ThinPoolName_tmeta

    Replace Size with the size for your metadata segment. Replace VolumeGroupName with name of the volume group. Replace ThinPoolName with the name of the thin pool.

Verification

  • Verify that the metadata_percent segment is extended:

    # lvs -o lv_name,seg_type,metadata_percent
    
      LV                Type      Meta%
      ThinPoolName      thin-pool 0.19

Additional resources

  • The lvs(8), lvextend(8) man pages
  • lvs -o help
4.3.3.2. Automatically extending a thin pool

You can automate the expansion of your thin pool by enabling monitoring and setting the thin_pool_autoextend_threshold and the thin_pool_autoextend_percent configuration parameters.

Prerequisites

  • Administrative access.

Procedure

  1. Check if the thin pool is monitored:

    # lvs -o lv_name,vg_name,seg_monitor
    
      LV                VG              Monitor
      ThinPoolName      VolumeGroupName not monitored
  2. Enable thin pool monitoring with the dmeventd daemon:

    # lvchange --monitor y VolumeGroupName/ThinPoolName

    Replace VolumeGroupName with the name of the volume group. Replace ThinPoolName with the name of the thin pool.

  3. As the root user, open the /etc/lvm/lvm.conf file in an editor of your choice.
  4. Uncomment the thin_pool_autoextend_threshold and thin_pool_autoextend_percent lines and set each parameter to a required value:

    thin_pool_autoextend_threshold = 70
    thin_pool_autoextend_percent = 20

    thin_pool_autoextend_threshold determines the percentage at which LVM starts to auto-extend the thin pool. For example, setting it to 70 means LVM will try to extend the thin pool when it reaches 70% capacity.

    thin_pool_autoextend_percent specifies by what percentage the thin pool should be extended when it reaches threshold. For example, setting it to 20 means the thin pool will be increased by 20% of its current size.

  5. Save the changes and exit the editor.
  6. Restart the lvm2-monitor:

    # systemctl restart lvm2-monitor

Additional resources

  • The lvs(8), lvchange(8), dmeventd(8) man pages

4.3.4. Shrinking logical volumes

When the size of the LV is reduced, the freed up logical extents are returned to the volume group and then can be used by other LVs.

Warning

Data stored in the reduced area is lost. Always back up the data and resize the file system before proceeding.

Prerequisites

  • Administrative access.

Procedure

  1. List the logical volumes and their volume groups:

    # lvs -o lv_name,vg_name,lv_size
    
      LV                   VG              LSize
      LogicalVolumeName    VolumeGroupName 6.49g
  2. Check where the logical volume is mounted:

    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/LogicalVolumeName
    
    SOURCE                                           TARGET
    /dev/mapper/VolumeGroupName-NewLogicalVolumeName /MountPoint

    Replace /dev/VolumeGroupName/LogicalVolumeName with the path to your logical volume.

  3. Unmount the logical volume:

    # umount /MountPoint

    Replace /MountPoint with the mounting point for your logical volume.

  4. Check and repair any file system errors:

    # e2fsck -f /dev/VolumeGroupName/LogicalVolumeName
  5. Resize the LV and the file system:

    # lvreduce --size TargetSize --resizefs VolumeGroupName/LogicalVolumeName

    Replace TargetSize with the new size of the LV. Replace VolumeGroupName/LogicalVolumeName with the path to your logical volume.

  6. Remount the file system:

    # mount -o remount /MountPoint

    Replace /MountPoint with the mounting point for your file system.

Verification

  1. Verify the space usage of the file system:

    # df -hT /MountPoint/
    
    Filesystem                                       Type  Size  Used Avail Use% Mounted on
    /dev/mapper/VolumeGroupName-NewLogicalVolumeName ext4  2.9G  139K  2.7G   1% /MountPoint

    Replace /MountPoint with the mounting point for your logical volume.

  2. Verify the size of the LV:

    # lvs -o lv_name,lv_size
    
      LV                   LSize
      NewLogicalVolumeName 4.00g

4.4. Renaming logical volumes

You can rename an existing logical volume, including snapshots, using the lvrename command.

Prerequisites

  • Administrative access.

Procedure

  1. List the logical volumes and their volume groups:

    # lvs -o lv_name,vg_name
    
      LV                VG
      LogicalVolumeName VolumeGroupName
  2. Rename the logical volume:

    # lvrename VolumeGroupName/LogicalVolumeName VolumeGroupName/NewLogicalVolumeName

    Replace VolumeGroupName with name of the volume group. Replace LogicalVolumeName with the name of the logical volume. Replace NewLogicalVolumeName with the new logical volume name.

Verification

  • Verify that the logical volume is renamed:

    # lvs -o lv_name
      LV
      NewLogicalVolumeName

Additional resources

  • lvrename(8) man page on your system

4.5. Removing logical volumes

You can remove an existing logical volume, including snapshots, using the lvremove command.

Prerequisites

  • Administrative access.

Procedure

  1. List the logical volumes and their paths:

    # lvs -o lv_name,lv_path
    
      LV                Path
      LogicalVolumeName /dev/VolumeGroupName/LogicalVolumeName
  2. Check where the logical volume is mounted:

    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/LogicalVolumeName
    
    SOURCE                                        TARGET
    /dev/mapper/VolumeGroupName-LogicalVolumeName /MountPoint

    Replace /dev/VolumeGroupName/LogicalVolumeName with the path to your logical volume.

  3. Unmount the logical volume:

    # umount /MountPoint

    Replace /MountPoint with the mounting point for your logical volume.

  4. Remove the logical volume:

    # lvremove VolumeGroupName/LogicalVolumeName

    Replace VolumeGroupName/LogicalVolumeName with the path to your logical volume.

Additional resources

  • lvs(8), lvremove(8) man pages on your system

4.6. Activating logical volumes

You can activate the logical volume with the lvchange command.

Prerequisites

  • Administrative access.

Procedure

  1. List the logical volumes, their volume groups, and their paths:

    # lvs -o lv_name,vg_name,lv_path
    
      LV                VG              Path
      LogicalVolumeName VolumeGroupName VolumeGroupName/LogicalVolumeName
  2. Activate the logical volume:

    # lvchange --activate y VolumeGroupName/LogicalVolumeName

    Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

    Note

    When activating a thin LV that was created as a snapshot of another LV, you might need to use the --ignoreactivationskip option to activate it.

Verification

  • Verify that the LV is active:

    # lvdisplay VolumeGroupName/LogicalVolumeName
    
      ...
      LV Status              available

    Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

Additional resources

  • The lvs(8) lvchange(8), lvdisplay(8) man pages

4.7. Deactivating logical volumes

By default, when you create a logical volume, it is in an active state. You can deactivate the logical volume with the lvchange command.

Warning

Deactivating a logical volume with active mounts or in use can lead to data inconsistencies and system errors.

Prerequisites

  • Administrative access.

Procedure

  1. List the logical volumes, their volume groups, and their paths:

    # lvs -o lv_name,vg_name,lv_path
    
      LV                VG              Path
      LogicalVolumeName VolumeGroupName /dev/VolumeGroupName/LogicalVolumeName
  2. Check where the logical volume is mounted:

    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/LogicalVolumeName
    
    SOURCE                                        TARGET
    /dev/mapper/VolumeGroupName-LogicalVolumeName /MountPoint

    Replace /dev/VolumeGroupName/LogicalVolumeName with the path to your logical volume.

  3. Unmount the logical volume:

    # umount /MountPoint

    Replace /MountPoint with the mounting point for your logical volume.

  4. Deactivate the logical volume:

    # lvchange --activate n VolumeGroupName/LogicalVolumeName

    Replace VolumeGroupName with name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

Verification

  • Verify that the LV is not active:

    # lvdisplay VolumeGroupName/LogicalVolumeName
    
      ...
      LV Status              NOT available

    Replace VolumeGroupName with name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

Additional resources

  • The lvs(8) lvchange(8), lvdisplay(8) man pages

Chapter 5. Advanced logical volume management

LVM includes advanced features such as:

  • Snapshots, which are point-in-time copies of logical volumes (LVs)
  • Caching, with which you can use faster storage as a cache for slower storage
  • Creating custom thin pools

5.1. Managing logical volume snapshots

A snapshot is a logical volume (LV) that mirrors the content of another LV at a specific point in time.

5.1.1. Understanding logical volume snapshots

When you create a snapshot, you are creating a new LV that serves as a point-in-time copy of another LV. Initially, the snapshot LV contains no actual data. Instead, it references the data blocks of the original LV at the moment of snapshot creation.

Warning

It is important to regularly monitor the snapshot’s storage usage. If a snapshot reaches 100% of its allocated space, it will become invalid.

It is essential to extend the snapshot before it gets completely filled. This can be done manually by using the lvextend command or automatically via the /etc/lvm/lvm.conf file.

Thick LV snapshots
When data on the original LV changes, the copy-on-write (CoW) system copies the original, unchanged data to the snapshot before the change is made. This way, the snapshot grows in size only as changes occur, storing the state of the original volume at the time of the snapshot’s creation. Thick snapshots are a type of LV that requires you to allocate some amount of storage space upfront. This amount can later be extended or reduced, however, you should consider what type of changes you intend to make to the original LV. This helps you to avoid either wasting resources by allocating too much space or needing to frequently increase the snapshot size if you allocate too little.
Thin LV snapshots

Thin snapshots are a type of LV created from an existing thin provisioned LV. Thin snapshots do not require allocating extra space upfront. Initially, both the original LV and its snapshot share the same data blocks. When changes are made to the original LV, it writes new data to different blocks, while the snapshot continues to reference the original blocks, preserving a point-in-time view of the LV’s data at the snapshot creation.

Thin provisioning is a method of optimizing and managing storage efficiently by allocating disk space on an as-needed basis. This means that you can create multiple LVs without needing to allocate a large amount of storage upfront for each LV. The storage is shared among all LVs in a thin pool, making it a more efficient use of resources. A thin pool allocates space on-demand to its LVs.

Choosing between thick and thin LV snapshots
The choice between thick or thin LV snapshots is directly determined by the type of LV you are taking a snapshot of. If your original LV is a thick LV, your snapshots will be thick. If your original LV is thin, your snapshots will be thin.

5.1.2. Managing thick logical volume snapshots

When you create a thick LV snapshot, it is important to consider the storage requirements and the intended lifespan of your snapshot. You need to allocate enough storage for it based on the expected changes to the original volume. The snapshot must have a sufficient size to capture changes during its intended lifespan, but it cannot exceed the size of the original LV. If you expect a low rate of change, a smaller snapshot size of 10%-15% might be sufficient. For LVs with a high rate of change, you might need to allocate 30% or more.

Important

It is essential to extend the snapshot before it gets completely filled. If a snapshot reaches 100% of its allocated space, it becomes invalid. You can monitor the snapshot capacity with the lvs -o lv_name,data_percent,origin command.

5.1.2.1. Creating thick logical volume snapshots

You can create a thick LV snapshot with the lvcreate command.

Prerequisites

Procedure

  1. Identify the LV of which you want to create a snapshot:

    # lvs -o vg_name,lv_name,lv_size
    
      VG              LV                LSize
      VolumeGroupName LogicalVolumeName 10.00g

    The size of the snapshot cannot exceed the size of the LV.

  2. Create a thick LV snapshot:

    # lvcreate --snapshot --size SnapshotSize --name SnapshotName VolumeGroupName/LogicalVolumeName

    Replace SnapshotSize with the size you want to allocate for the snapshot (e.g. 10G). Replace SnapshotName with the name you want to give to the snapshot logical volume. Replace VolumeGroupName with the name of the volume group that contains the original logical volume. Replace LogicalVolumeName with the name of the logical volume that you want to create a snapshot of.

Verification

  • Verify that the snapshot is created:

    # lvs -o lv_name,origin
    
      LV                  Origin
      LogicalVolumeName
      SnapshotName        LogicalVolumeName

Additional resources

  • lvcreate(8) and lvs(8) man pages
5.1.2.2. Manually extending logical volume snapshots

If a snapshot reaches 100% of its allocated space, it becomes invalid. It is essential to extend the snapshot before it gets completely filled. This can be done manually by using the lvextend command.

Prerequisites

  • Administrative access.

Procedure

  1. List the names of volume groups, logical volumes, source volumes for snapshots, their usage percentages, and sizes:

    # lvs -o vg_name,lv_name,origin,data_percent,lv_size
      VG              LV                Origin            Data%  LSize
      VolumeGroupName LogicalVolumeName                          10.00g
      VolumeGroupName SnapshotName      LogicalVolumeName 82.00   5.00g
  2. Extend the thick-provisioned snapshot:

    # lvextend --size +AdditionalSize VolumeGroupName/SnapshotName

    Replace AdditionalSize with how much space to add to the snapshot (for example, +1G). Replace VolumeGroupName with the name of the volume group. Replace SnapshotName with the name of the snapshot.

Verification

  • Verify that the LV is extended:

    # lvs -o vg_name,lv_name,origin,data_percent,lv_size
      VG              LV                Origin            Data%  LSize
      VolumeGroupName LogicalVolumeName                          10.00g
      VolumeGroupName SnapshotName      LogicalVolumeName 68.33   6.00g
5.1.2.3. Automatically extending thick logical volume snapshots

If a snapshot reaches 100% of its allocated space, it becomes invalid. It is essential to extend the snapshot before it gets completely filled. This can be done automatically.

Prerequisites

  • Administrative access.

Procedure

  1. As the root user, open the /etc/lvm/lvm.conf file in an editor of your choice.
  2. Uncomment the snapshot_autoextend_threshold and snapshot_autoextend_percent lines and set each parameter to a required value:

    snapshot_autoextend_threshold = 70
    snapshot_autoextend_percent = 20

    snapshot_autoextend_threshold determines the percentage at which LVM starts to auto-extend the snapshot. For example, setting the parameter to 70 means that LVM will try to extend the snapshot when it reaches 70% capacity.

    snapshot_autoextend_percent specifies by what percentage the snapshot should be extended when it reaches the threshold. For example, setting the parameter to 20 means the snapshot will be increased by 20% of its current size.

  3. Save the changes and exit the editor.
  4. Restart the lvm2-monitor:

    # systemctl restart lvm2-monitor
5.1.2.4. Merging thick logical volume snapshots

You can merge thick LV snapshot into the original logical volume from which the snapshot was created. The process of merging means that the original LV is reverted to the state it was in when the snapshot was created. Once the merge is complete, the snapshot is removed.

Note

The merge between the original and snapshot LV is postponed if either is active. It only proceeds once the LVs are reactivated and not in use.

Prerequisites

  • Administrative access.

Procedure

  1. List the LVs, their volume groups, and their paths:

    # lvs -o lv_name,vg_name,lv_path
    
      LV                   VG              Path
      LogicalVolumeName    VolumeGroupName /dev/VolumeGroupName/LogicalVolumeName
      SnapshotName         VolumeGroupName /dev/VolumeGroupName/SnapshotName
  2. Check where the LVs are mounted:

    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/LogicalVolumeName
    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/SnapshotName

    Replace /dev/VolumeGroupName/LogicalVolumeName with the path to your logical volume. Replace /dev/VolumeGroupName/SnapshotName with the path to your snapshot.

  3. Unmount the LVs:

    # umount /LogicalVolume/MountPoint
    # umount /Snapshot/MountPoint

    Replace /LogicalVolume/MountPoint with the mounting point for your logical volume. Replace /Snapshot/MountPoint with the mounting point for your snapshot.

  4. Deactivate the LVs:

    # lvchange --activate n VolumeGroupName/LogicalVolumeName
    # lvchange --activate n VolumeGroupName/SnapshotName

    Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume. Replace SnapshotName with the name of your snapshot.

  5. Merge the thick LV snapshot into the origin:

    # lvconvert --merge SnapshotName

    Replace SnapshotName with the name of the snapshot.

  6. Activate the LV:

    # lvchange --activate y VolumeGroupName/LogicalVolumeName

    Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

  7. Mount the LV:

    # umount /LogicalVolume/MountPoint

    Replace /LogicalVolume/MountPoint with the mounting point for your logical volume.

Verification

  • Verify that the snapshot is removed:

    # lvs -o lv_name

Additional resources

  • The lvconvert(8), lvs(8) man page

5.1.3. Managing thin logical volume snapshots

Thin provisioning is appropriate where storage efficiency is a priority. Storage space dynamic allocation reduces initial storage costs and maximizes the use of available storage resources. In environments with dynamic workloads or where storage grows over time, thin provisioning allows for flexibility. It enables the storage system to adapt to changing needs without requiring large upfront allocations of the storage space. With dynamic allocation, over-provisioning is possible, where the total size of all LVs can exceed the physical size of the thin pool, under the assumption that not all space will be utilized at the same time.

5.1.3.1. Creating thin logical volume snapshots

You can create a thin LV snapshot with the lvcreate command. When creating a thin LV snapshot, avoid specifying the snapshot size. Including a size parameter results in the creation of a thick snapshot instead.

Prerequisites

Procedure

  1. Identify the LV of which you want to create a snapshot:

    # lvs -o lv_name,vg_name,pool_lv,lv_size
    
      LV                VG              Pool       LSize
      PoolName          VolumeGroupName            152.00m
      ThinVolumeName    VolumeGroupName PoolName   100.00m
  2. Create a thin LV snapshot:

    # lvcreate --snapshot --name SnapshotName VolumeGroupName/ThinVolumeName

    Replace SnapshotName with the name you want to give to the snapshot logical volume. Replace VolumeGroupName with the name of the volume group that contains the original logical volume. Replace ThinVolumeName with the name of the thin logical volume that you want to create a snapshot of.

Verification

  • Verify that the snapshot is created:

    # lvs -o lv_name,origin
    
      LV                Origin
      PoolName
      SnapshotName      ThinVolumeName
      ThinVolumeName

Additional resources

  • lvcreate(8) and lvs(8) man pages
5.1.3.2. Merging thin logical volume snapshots

You can merge thin LV snapshot into the original logical volume from which the snapshot was created. The process of merging means that the original LV is reverted to the state it was in when the snapshot was created. Once the merge is complete, the snapshot is removed.

Prerequisites

  • Administrative access.

Procedure

  1. List the LVs, their volume groups, and their paths:

    # lvs -o lv_name,vg_name,lv_path
    
      LV                VG              Path
      ThinPoolName      VolumeGroupName
      ThinSnapshotName  VolumeGroupName /dev/VolumeGroupName/ThinSnapshotName
      ThinVolumeName    VolumeGroupName /dev/VolumeGroupName/ThinVolumeName
  2. Check where the original LV is mounted:

    # findmnt -o SOURCE,TARGET /dev/VolumeGroupName/ThinVolumeName

    Replace VolumeGroupName/ThinVolumeName with the path to your logical volume.

  3. Unmount the LV:

    # umount /ThinLogicalVolume/MountPoint

    Replace /ThinLogicalVolume/MountPoint with the mounting point for your logical volume. Replace /ThinSnapshot/MountPoint with the mounting point for your snapshot.

  4. Deactivate the LV:

    # lvchange --activate n VolumeGroupName/ThinLogicalVolumeName

    Replace VolumeGroupName with the name of the volume group. Replace ThinLogicalVolumeName with the name of the logical volume.

  5. Merge the thin LV snapshot into the origin:

    # lvconvert --mergethin VolumeGroupName/ThinSnapshotName

    Replace VolumeGroupName with the name of the volume group. Replace ThinSnapshotName with the name of the snapshot.

  6. Mount the LV:

    # umount /ThinLogicalVolume/MountPoint

    Replace /ThinLogicalVolume/MountPoint with the mounting point for your logical volume.

Verification

  • Verify that the original LV is merged:

    # lvs -o lv_name

Additional resources

  • The lvremove(8), lvs(8) man page

5.2. Caching logical volumes

You can cache logical volumes by using the dm-cache or dm-writecache targets.

dm-cache utilizes faster storage device (SSD) as cache for a slower storage device (HDD). It caches read and write data, optimizing access times for frequently used data. It is beneficial in mixed workload environments where enhancing read and write operations can lead to significant performance improvements.

dm-writecache optimizes write operations by using a faster storage medium (SSD) to temporarily hold write data before it is committed to the primary storage device (HDD). It is beneficial for write-intensive applications where write performance can slow down the data transfer process.

5.2.1. Caching logical volumes with dm-cache

When caching LV with dm-cache, a cache pool is created. A cache pool is a LV that combines both the cache data, which stores the actual cached content, and cache metadata, which tracks what content is stored in the cache. This pool is then associated with a specific LV to cache its data.

dm-cache targets two types of blocks: frequently accessed (hot) blocks are moved to the cache, while less frequently accessed (cold) blocks remain on the slower device.

Prerequisites

  • Administrative access.

Procedure

  1. Display the LV you want to cache and its volume group:

    # lvs -o lv_name,vg_name
      LV                   VG
      LogicalVolumeName    VolumeGroupName
  2. Create the cache pool:

    # lvcreate --type cache-pool --name CachePoolName --size Size VolumeGroupName /FastDevicePath

    Replace CachePoolName with the name of the cache pool. Replace Size with the size for your cache pool. Replace VolumeGroupName with the name of the volume group. Replace /FastDevicePath with the path to your fast device, for example SSD or NVME.

  3. Attach the cache pool to the LV:

    # lvconvert --type cache --cachepool VolumeGroupName/CachePoolName VolumeGroupName/LogicalVolumeName

Verification

  • Verify that the LV is now cached:

    # lvs -o lv_name,pool_lv
    
      LV                   Pool
      LogicalVolumeName    [CachePoolName_cpool]

Additional resources

  • lvcreate(8), lvconvert(8), lvs(8) man pages

5.2.2. Caching logical volumes with dm-writecache

When caching LVs with dm-writecache, a caching layer between the logical volume and the physical storage device is created. dm-writecache operates by temporarily storing write operations in a faster storage medium, such as an SSD, before eventually writing them back to the primary storage device, optimizing write-intensive workloads.

Prerequisites

  • Administrative access.

Procedure

  1. Display the logical volume you want to cache and its volume group:

    # lvs -o lv_name,vg_name
      LV                   VG
      LogicalVolumeName    VolumeGroupName
  2. Create a cache volume:

    # lvcreate --name CacheVolumeName --size Size VolumeGroupName /FastDevicePath

    Replace CacheVolumeName with the name of the cache volume. Replace Size with the size for your cache pool. Replace VolumeGroupName with the name of the volume group. Replace /FastDevicePath with the path to your fast device, for example SSD or NVME.

  3. Attach the cache volume to the LV:

    # lvconvert --type writecache --cachevol CacheVolumeName VolumeGroupName/LogicalVolumeName

    Replace CacheVolumeName with the name of the cache volume. Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

Verification

  • Verify that the LV is now cached:

    # lvs -o lv_name,pool_lv
    
      LV                   Pool
      LogicalVolumeName    [CacheVolumeName_cvol]

Additional resources

  • lvcreate(8), lvconvert(8), lvs(8) man pages

5.2.3. Uncaching a logical volume

Use two main ways to remove caching from a LV.

Splitting
You can detach the cache from the LV but preserve the cache volume itself. In this case the LV will no longer benefit from the caching mechanism but the cache volume and its data will remain intact. While the cache volume is preserved, the data within the cache cannot be reused and will be erased the next time it is used in a caching setup.
Uncaching
You can detaches the cache from the LV and remove the cache volume entirely. This action effectively destroys the cache, freeing up the space.

Prerequisites

  • Administrative access.

Procedure

  1. Display the cached LV:

    # lvs -o lv_name,pool_lv,vg_name
    
      LV                   Pool                   VG
      LogicalVolumeName    [CacheVolumeName_cvol] VolumeGroupName
  2. Detach or remove the cached volume:

    • To detach the cached volume, use:

      # lvconvert --splitcache VolumeGroupName/LogicalVolumeName
    • To detach and remove the cached volume, use:

      # lvconvert --uncache VolumeGroupName/LogicalVolumeName

      Replace VolumeGroupName with the name of the volume group. Replace LogicalVolumeName with the name of the logical volume.

Verification

  • Verify that the LV is not cached:

    # lvs -o lv_name,pool_lv

Additional resources

  • lvconvert(8), lvs(8) man pages

5.3. Creating a custom thin pool

You can create custom thin pools to have a better control over the storage.

Prerequisites

  • Administrative access.

Procedure

  1. Display available volume groups:

    # vgs -o vg_name
    
      VG
      VolumeGroupName
  2. List available devices:

    # lsblk
  3. Create a LV to hold the thin pool data:

    # lvcreate --name ThinPoolDataName --size Size VolumeGroupName /DevicePath

    Replace ThinPoolDataName with the name for your thin pool data LV. Replace Size with the size for your LV. Replace VolumeGroupName with the name of your volume group.

  4. Create a LV to hold the thin pool metadata:

    # lvcreate --name ThinPoolMetadataName --size Size VolumeGroupName /DevicePath
  5. Combine the LVs into a thin pool:

    # lvconvert --type thin-pool --poolmetadata ThinPoolMetadataName VolumeGroupName/ThinPoolDataName

Verification

  1. Verify that the custom thin pool is created:

    # lvs -o lv_name,seg_type
    
      LV                Type
      ThinPoolDataName  thin-pool

Additional resources

  • The vgs(8) lvs(8), lvcreate(8) man pages

Chapter 6. Customizing the LVM report

LVM provides a wide range of configuration and command line options to produce customized reports and to filter the report’s output. You can sort the output, specify units, use selection criteria, and update the lvm.conf file to customize the LVM report.

6.1. Controlling format of the LVM display

Whether you use pvs, lvs, or vgs, these commands determine the default set of fields displayed and the sort order. You can control the output of these commands by executing the following commands.

Procedure

  • Change the default fields in the LVM display using the -o option:

    # pvs -o pv_name,pv_size,pv_free
      PV         PSize  PFree
      /dev/vdb1  17.14G 17.14G
      /dev/vdb2  17.14G 17.09G
      /dev/vdb3  17.14G 17.14G
  • Sort LVM display by using the -O option:

    # pvs -o pv_name,pv_size,pv_free -O pv_free
      PV         PSize  PFree
      /dev/vdb2 17.14G 17.09G
      /dev/vdb1  17.14G 17.14G
      /dev/vdb3  17.14G 17.14G
  • Display a reverse sort by using the -O argument along with the - character:

    # pvs -o pv_name,pv_size,pv_free -O -pv_free
      PV         PSize  PFree
      /dev/vdb1  17.14G 17.14G
      /dev/vdb3  17.14G 17.14G
      /dev/vdb2  17.14G 17.09G

Additional resources

6.2. Specifying the units for an LVM report display

You can view the size of the LVM devices in base 2 or base 10 units by specifying the --units argument of the report command.

Base 2 units

The default units are displayed in powers of 2, which is multiples of 1024. You can specify it using human-readable (r) with < and > rounding indicator, bytes (b), sectors (s), kilobytes (k), megabytes (m), gigabytes (g), terabytes (t), petabytes (p), exabytes (e), and human-readable (h).

The default display is r, when --units is not specified. You can override the default by setting the units parameter in the global section of the /etc/lvm/lvm.conf file.

Base 10 units
You can specify the units to be displayed in multiples of 1000 by capitalizing the unit specification (R, B, S, K, M, G, T, P, E, H).

Procedure

  • Specify the units for the LVM for base 2 gigabytes units:

    # pvs --units g /dev/vdb
      PV        VG    Fmt  Attr PSize   PFree
      /dev/vdb  myvg  lvm2 a--  931.00g 930.00g
    
    # vgs --units g myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 931.00g 931.00g
    
    # lvs --units g myvg
      LV    VG   Attr     LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      mylv myvg  wi-a---- 1.OOg
  • Indicate the actual size of LVM by using the r option with the < or > prefix in the output:

    # vgs --units g myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 931.00g 930.00g
    
    # vgs --units r myvg
      VG   #PV #LV #SN Attr VSize    VFree
      myvg   1   1   0 wz-n <931.00g <930.00
    
    # vgs myvg
      VG   #PV #LV #SN Attr VSize    VFree
      myvg   1   1   0 wz-n <931.00g <930.00g

    The r unit works similarly to h (human-readable), but in addition, the reported value gets a prefix of < or > to indicate that the actual size is slightly more or less than the displayed size. LVM rounds the decimal value, causing non-exact sizes to be reported.

    It also shows how --units g or other --units do not always display exactly correct sizes. It also shows the primary purpose of r, which is the < to indicate that the displayed size is not exact. In this example, the value is not exact because the VG size is not an exact multiple of gigabytes, and .01 is also not an exact representation of the fraction.

  • Specify the units for the LVM for base 10 gigabytes units:

    # pvs --units G /dev/vdb
      PV        VG   Fmt  Attr  PSize   PFree
      /dev/vdb  myvg lvm2 a--   999.65G 998.58G
    
    # vgs --units G myvg
      VG   #PV #LV #SN Attr VSize   VFree
      myvg   1   1   0 wz-n 999.65G 998.58G
    
    # lvs --units G myvg
      LV    VG   Attr     LSize Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      mylv myvg wi-a---- 1.07G
  • Specify sectors (s), defined as 512 bytes, or custom units. The following example displays the output of the pvs command as several sectors:

    # pvs --units s
      PV         VG     Fmt  Attr PSize       PFree
      /dev/vdb   myvg   lvm2 a--  1952440320S 1950343168S
  • Specify megabytes (m). The following example displays the output of the pvs command in units of 4 MB:

    # pvs --units 4m
      PV         VG     Fmt  Attr PSize      PFree
      /dev/vdb   myvg   lvm2 a--  238335.00U 238079.00U

6.3. Customizing the LVM configuration file

By editing the lvm.conf file, you can customize the LVM according to your specific storage and system requirements. For example, you can use lvm.conf to modify filter settings, configure volume group auto activation, manage thin pool, or automatically extend a snapshot.

Procedure:

  1. Display the default lvm.conf file:

    # lvmconfig --typeconfig default --withcomments

    By default, the lvm.conf file contains only comments to display possible settings.

  2. Customize the lvm.conf file according to your requirements by uncommenting the setting in lvm.conf. The following setting focuses on changing the default display of certain commands:

    • In the lvm.conf file, adjust the lvs_cols parameter to only print the specified fields:

      {
        ...
        lvs_cols="lv_name,vg_name,lv_attr"
        ...
      }

      Use this option instead of the lvs -o lv_name,vg_name,lv_attr command to avoid unnecessary frequent use of the -o option.

    • In the lvm.conf file, use the compact_output=1 setting to avoid printing empty fields for the pvs, vgs, and lvs commands:

      {
        ...
        compact_output = 1
        ...
      }
  3. View the default values after modifying the lvm.conf file:

    # lvmconfig --typeconfig diff

Additional resources

  • lvm.conf(5) man page on your system

6.4. Defining LVM selection criteria

Selection criteria are a set of statements in the form of <field> <operator> <value>, which use comparison operators to define values for specific fields. Objects that match the selection criteria are then processed or displayed. Statements are combined by logical and grouping operators. To define selection criteria use the -S or --select option followed by one or multiple statements.

Some LVM commands support the -S option to select which objects to process based on certain attributes. These objects can be physical volumes (PVs), volume groups (VGs), or logical volumes (LVs).

The -S option works by describing the objects to process, rather than naming each object. This is helpful when processing many objects and it would be difficult to find and name each object separately or when searching objects that have a complex set of characteristics. The select option can also be used as a shortcut to avoid typing many names.

Use the lvs -S help command to see full sets of fields and possible operators. Replace lvs with any reporting or processing command to see the details of that command.

Use selection criteria with LVM reporting and processing commands to only display or process the objects that satisfy chosen criteria:

  • Reporting commands include pvs, vgs, lvs, pvdisplay, vgdisplay, lvdisplay, and dmsetup info -c.
  • Processing commands include pvchange, vgchange, lvchange, vgimport, vgexport, vgremove, and lvremove.

Procedure

  • Examples of selection criteria using the pvs command:

    # pvs
      PV           VG   Fmt  Attr PSize    PFree
      /dev/nvme2n1       lvm2 ---     1.00g   1.00g
      /dev/vdb1     myvg lvm2 a--  1020.00m 396.00m
      /dev/vdb2     myvg lvm2 a--  1020.00m 896.00m
    # pvs -S name=~nvme
      PV           Fmt  Attr PSize PFree
      /dev/nvme2n1 lvm2 ---  1.00g 1.00g
    # pvs -S vg_name=myvg
      PV         VG   Fmt  Attr PSize    PFree
      /dev/vdb1   myvg lvm2 a--  1020.00m 396.00m
      /dev/vdb2   myvg lvm2 a--  1020.00m 896.00m
  • Examples of selection criteria using the lvs commands:

    # lvs
      LV    VG   Attr       LSize   Cpy%Sync
      mylv  myvg -wi-a----- 200.00m
      lvol0 myvg -wi-a----- 100.00m
      lvol1 myvg -wi-a----- 100.00m
      lvol2 myvg -wi------- 100.00m
      rr    myvg rwi-a-r--- 120.00m 100.00
    # lvs -S 'size > 100m && size < 200m'
      LV   VG   Attr       LSize   Cpy%Sync
      rr   myvg rwi-a-r--- 120.00m 100.00
    # lvs -S name=~lvol[02]
      LV    VG   Attr       LSize
      lvol0 myvg -wi-a----- 100.00m
      lvol2 myvg -wi------- 100.00m
    # lvs -S segtype=raid1
      LV   VG   Attr       LSize   Cpy%Sync
      rr   myvg rwi-a-r--- 120.00m 100.00
  • More advanced examples:

    # lvchange --addtag mytag -S active=1
      Logical volume myvg/mylv changed.
      Logical volume myvg/lvol0 changed.
      Logical volume myvg/lvol1 changed.
      Logical volume myvg/rr changed.
    # lvs -a -o lv_name,vg_name,attr,size,pool_lv,origin,role -S 'name!~_pmspare'
      LV         VG      Attr       LSize Pool Origin Role
      thin1      example Vwi-a-tz-- 2.00g tp          public,origin,thinorigin
      thin1s     example Vwi---tz-- 2.00g tp   thin1  public,snapshot,thinsnapshot
      thin2      example Vwi-a-tz-- 3.00g tp          public
      tp         example twi-aotz-- 1.00g             private
      [tp_tdata] example Twi-ao---- 1.00g             private,thin,pool,data
      [tp_tmeta] example ewi-ao---- 4.00m             private,thin,pool,metadata
    # lvchange --setactivationskip n -S 'role=thinsnapshot && origin=thin1'
      Logical volume myvg/thin1s changed.
    # lvs -a -S 'name=~_tmeta && role=metadata && size <= 4m'
      LV         VG      Attr       LSize
      [tp_tmeta] myvg   ewi-ao---- 4.00m

Additional resources

  • lvmreport(7) man page on your system

6.5. Expressing pool volume sizes as percentage by using the storage RHEL system role

The example Ansible playbook applies the storage system role to enable you to express Logical Manager Volumes (LVM) volume sizes as a percentage of the pool’s total size.

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: Express volume sizes as a percentage of the pool's total size
      hosts: managed-node-01.example.com
      roles:
        - rhel-system-roles.storage
      vars:
        storage_pools:
          - name: myvg
            disks:
              - /dev/sdb
            volumes:
              - name: data
                size: 60%
                mount_point: /opt/mount/data
              - name: web
                size: 30%
                mount_point: /opt/mount/web
              - name: cache
                size: 10%
                mount_point: /opt/cache/mount

    This example specifies the size of LVM volumes as a percentage of the pool size, for example: 60%. Alternatively, you can also specify the size of LVM volumes as a percentage of the pool size in a human-readable size of the file system, for example, 10g or 50 GiB.

  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.storage/README.md file
  • /usr/share/doc/rhel-system-roles/storage/ directory

Chapter 7. Configuring LVM on shared storage

Shared storage is storage that can be accessed by multiple nodes at the same time. You can use LVM to manage shared storage. Shared storage is commonly used in cluster and high-availability setups and there are two common scenarios for how shared storage appears on the system:

  • LVM devices are attached to a host and passed to a guest VM to use. In this case, the device is never intended to be used by the host, only by the guest VM.
  • Machines are attached to a storage area network (SAN), for example using Fiber Channel, and the SAN LUNs are visible to multiple machines:

7.1. Configuring LVM for VM disks

To prevent VM storage from being exposed to the host, you can configure LVM device access and LVM system ID. You can do this by excluding the devices in question from the host, which ensures that the LVM on the host doesn’t see or use the devices passed to the guest VM. You can protect against accidental usage of the VM’s VG on the host by setting the LVM system ID in the VG to match the guest VM.

Procedure

  1. In the lvm.conf file, filter the path to exclude the device:

    filter = [ "r|^path_to_device$|" ]
  2. Optional: You can further protect LVM devices:

    1. Set the LVM system ID feature in both the host and the VM in the lvm.conf file:

      system_id_source = "uname"
    2. Set the VG’s system ID to match the VM system ID. This ensures that only the guest VM is capable of activating the VG:

      $ vgchange --systemid <VM_system_id> <VM_vg_name>

7.2. Configuring LVM to use SAN disks on one machine

To prevent the SAN LUNs from being used by the wrong machine, exclude those disks in the lvm.conf filter on all machines except the one machine which is meant to use them.

You can also protect the VG from being used by the wrong machine by configuring a system ID on all machines, and setting the system ID in the VG to match the machine using it.

Procedure

  1. In the lvm.conf file filter the path to the device to exclude it:

    filter = [ "r|^path_to_device$|" ]
  2. Set the LVM system ID feature in the lvm.conf file:

    system_id_source = "uname"
  3. Set the VG’s system ID to match the system ID of the machine using this VG:

    $ vgchange --systemid <system_id> <vg_name>

7.3. Configuring LVM to use SAN disks for failover

You can configure LUNs to be moved between machines, for example for failover purposes. You can set up the LVM by configuring the lvm.conf filter to include the LUNs on all machines that may use them and by configuring the LVM system ID on each machine.

The following procedure describes the initial LVM configuration, to finish setting up LVM for failover and move the VG between machines, you need to configure pacemaker and LVM-activate resource agent that will automatically modify the VG’s system ID to match the system ID of the machine where the VG can be used. For more information see Configuring and managing high availability clusters.

Procedure

  1. In the lvm.conf file, filter the path to exclude the device:

    filter = [ "a|^path_to_device$|" ]
  2. Set the LVM system ID feature in all machines in the lvm.conf file:

    system_id_source = "uname"

7.4. Configuring LVM to share SAN disks among multiple machines

Using the lvmlockd daemon and a lock manager such as dlm or sanlock, you can enable access to a shared VG on the SAN disks from multiple machines. The specific commands may differ based on the lock manager and operating system used. The following procedure describes the overview of the required steps to configure LVM to share SAN disks among multiple machines.

Warning

When using pacemaker, the system must be configured and started using the pacemaker steps shown in Configuring and managing high availability clusters instead.

Procedure

  1. Configure the lvm.conf filter to include the LUNs of all machines that will use them:

    filter = ["a|^path_to_device$|" ]
  2. Configure the lvm.conf file to use the lvmlockd daemon on all machines:

    use_lvmlockd=1
  3. Start the lvmlockd daemon file on all machines.
  4. Start a lock manager to use with lvmlockd, such as dlm or sanlock on all machines.
  5. Create a new shared VG using the command vgcreate --shared.
  6. Start and stop access to existing shared VGs using the commands vgchange --lockstart and vgchange --lockstop on all machines.

Additional resources

  • lvmlockd(8) man page on your system

7.5. Creating shared LVM devices using the storage RHEL system role

You can use the storage RHEL system role to create shared LVM devices if you want your multiple systems to access the same storage at the same time.

This can bring the following notable benefits:

  • Resource sharing
  • Flexibility in managing storage resources
  • Simplification of storage management tasks

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: Create shared LVM device
      hosts: managed-node-01.example.com
      become: true
      tasks:
        - name: Create shared LVM device
          ansible.builtin.include_role:
            name: rhel-system-roles.storage
          vars:
            storage_pools:
              - name: vg1
                disks: /dev/vdb
                type: lvm
                shared: true
                state: present
                volumes:
                  - name: lv1
                    size: 4g
                    mount_point: /opt/test1
            storage_safe_mode: false
            storage_use_partitions: true
  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.storage/README.md file
  • /usr/share/doc/rhel-system-roles/storage/ directory

Chapter 8. Configuring RAID logical volumes

You can create and manage Redundant Array of Independent Disks (RAID) volumes by using logical volume manager (LVM).

8.1. RAID logical volumes

Logical volume manager (LVM) supports Redundant Array of Independent Disks (RAID) levels 0, 1, 4, 5, 6, and 10. An LVM RAID volume has the following characteristics:

  • LVM creates and manages RAID logical volumes that leverage the Multiple Devices (MD) kernel drivers.
  • You can temporarily split RAID1 images from the array and merge them back into the array later.
  • LVM RAID volumes support snapshots.

Other characteristics include:

Clusters

RAID logical volumes are not cluster-aware.

Although you can create and activate RAID logical volumes exclusively on one machine, you cannot activate them simultaneously on more than one machine.

Subvolumes

When you create a RAID logical volume (LV), LVM creates a metadata subvolume that is one extent in size for every data or parity subvolume in the array.

For example, creating a 2-way RAID1 array results in two metadata subvolumes (lv_rmeta_0 and lv_rmeta_1) and two data subvolumes (lv_rimage_0 and lv_rimage_1). Similarly, creating a 3-way stripe and one implicit parity device, RAID4 results in four metadata subvolumes (lv_rmeta_0, lv_rmeta_1, lv_rmeta_2, and lv_rmeta_3) and four data subvolumes (lv_rimage_0, lv_rimage_1, lv_rimage_2, and lv_rimage_3).

Integrity
You can lose data when a RAID device fails or when soft corruption occurs. Soft corruption in data storage implies that the data retrieved from a storage device is different from the data written to that device. Adding integrity to a RAID LV reduces or prevent soft corruption. For more information, see Creating a RAID LV with DM integrity.

8.2. RAID levels and linear support

The following are the supported configurations by RAID, including levels 0, 1, 4, 5, 6, 10, and linear:

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 stripes and written across the member disks of the array, allowing high I/O performance at low inherent cost but provides no redundancy.

RAID level 0 implementations 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 gets treated as though it was the same size as the smallest drive. Therefore, the common storage capacity of a level 0 array is the total capacity of all disks. If the member disks have a different size, then the RAID0 uses all the space of those disks using the available zones.

Level 1

RAID level 1, or mirroring, 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 relatively high costs.

RAID level 1 is costly because you write the same information to all of the disks in the array, which 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, which is parity-based RAID levels that 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.

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 concentrated on a single disk drive to protect data. Parity information is calculated based on the content 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.

Since 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 it is used in specific circumstances where the system administrator is intentionally designing the software RAID device with this bottleneck in mind such as an array that has 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 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. The performance of a RAID level 4 array is always asymmetrical, which means reads outperform writes. This is because write operations consume extra CPU resources and main memory bandwidth when generating parity, and then also consume extra bus bandwidth when writing the actual data to disks because you are not only writing the data, but also the parity. Read operations need only read the data and not the parity unless the array is in a degraded state. As a result, read operations generate less traffic to the drives and across the buses 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 the member disk drives of an array, RAID level 5 eliminates the write bottleneck inherent in level 4. The only performance bottleneck is the parity calculation process itself. Modern CPUs can calculate parity very fast. However, if you have a large number of disks in a RAID 5 array such that the combined aggregate data transfer speed across all devices is high enough, parity calculation can be a bottleneck.

Level 5 has asymmetrical performance, and 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 two devices instead of one 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 reduces some of the space wasted in level 1 arrays with more than two devices. With level 10, it is possible, for example, to create a 3-drive array configured to store only two 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, similar to a 3-device, level 1 array. This avoids CPU process usage to calculate parity similar to RAID level 6, but it is less space efficient.

The creation of RAID level 10 is not supported during installation. It is possible to create one manually after installation.

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 and data can be lost. The capacity is the total of all member disks.

8.3. LVM RAID segment types

To create a RAID logical volume, you can specify a RAID type by using the --type argument of the lvcreate command. For most users, specifying one of the five available primary types, which are raid1, raid4, raid5, raid6, and raid10, should be sufficient.

The following table describes the possible RAID segment types.

Table 8.1. LVM RAID segment types
Segment typeDescription

raid1

RAID1 mirroring. This is the default value for the --type argument of the lvcreate command, when you specify the -m argument without specifying striping.

raid4

RAID4 dedicated parity disk.

raid5_la

  • RAID5 left asymmetric.
  • Rotating parity 0 with data continuation.

raid5_ra

  • RAID5 right asymmetric.
  • Rotating parity N with data continuation.

raid5_ls

  • RAID5 left symmetric.
  • It is same as raid5.
  • Rotating parity 0 with data restart.

raid5_rs

  • RAID5 right symmetric.
  • Rotating parity N with data restart.

raid6_zr

  • RAID6 zero restart.
  • It is same as raid6.
  • Rotating parity zero (left-to-right) with data restart.

raid6_nr

  • RAID6 N restart.
  • Rotating parity N (left-to-right) with data restart.

raid6_nc

  • RAID6 N continue.
  • Rotating parity N (left-to-right) with data continuation.

raid10

  • Striped mirrors. This is the default value for the --type argument of the lvcreate command if you specify the -m argument along with the number of stripes that is greater than 1.
  • Striping of mirror sets.

raid0/raid0_meta

Striping. RAID0 spreads logical volume data across multiple data subvolumes in units of stripe size. This is used to increase performance. Logical volume data is lost if any of the data subvolumes fail.

8.4. Creating RAID logical volumes

You can create RAID1 arrays with multiple numbers of copies, according to the value you specify for the -m argument. Similarly, you can specify the number of stripes for a RAID 0, 4, 5, 6, and 10 logical volume with the -i argument. You can also specify the stripe size with the -I argument. The following procedure describes different ways to create different types of RAID logical volume.

Procedure

  • Create a 2-way RAID. The following command creates a 2-way RAID1 array, named my_lv, in the volume group my_vg, that is 1G in size:

    # lvcreate --type raid1 -m 1 -L 1G -n my_lv my_vg
    Logical volume "my_lv" created.
  • Create a RAID5 array with stripes. The following command creates a RAID5 array with three stripes and one implicit parity drive, named my_lv, in the volume group my_vg, that is 1G in size. Note that you can specify the number of stripes similar to an LVM striped volume. The correct number of parity drives is added automatically.

    # lvcreate --type raid5 -i 3 -L 1G -n my_lv my_vg
  • Create a RAID6 array with stripes. The following command creates a RAID6 array with three 3 stripes and two implicit parity drives, named my_lv, in the volume group my_vg, that is 1G one gigabyte in size:

    # lvcreate --type raid6 -i 3 -L 1G -n my_lv my_vg

Verification

  • Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:
# lvs -a -o name,copy_percent,devices _my_vg_
  LV                Copy%  Devices
  my_lv             6.25    my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]         /dev/sde1(0)
  [my_lv_rimage_1]         /dev/sdf1(1)
  [my_lv_rmeta_0]          /dev/sde1(256)
  [my_lv_rmeta_1]          /dev/sdf1(0)

Additional resources

  • lvcreate(8) and lvmraid(7) man pages on your system

8.5. Creating a RAID0 striped logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size. The following procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks.

Prerequisites

  1. You have created three or more physical volumes. For more information about creating physical volumes, see Creating LVM physical volume.
  2. You have created the volume group. For more information, see Creating LVM volume group.

Procedure

  1. Create a RAID0 logical volume from the existing volume group. The following command creates the RAID0 volume mylv from the volume group myvg, which is 2G in size, with three stripes and a stripe size of 4kB:

    # lvcreate --type raid0 -L 2G --stripes 3 --stripesize 4 -n mylv my_vg
      Rounding size 2.00 GiB (512 extents) up to stripe boundary size 2.00 GiB(513 extents).
      Logical volume "mylv" created.
  2. Create a file system on the RAID0 logical volume. The following command creates an ext4 file system on the logical volume:

    # mkfs.ext4 /dev/my_vg/mylv
  3. Mount the logical volume and report the file system disk space usage:

    # mount /dev/my_vg/mylv /mnt
    
    # df
    Filesystem             1K-blocks     Used  Available  Use% Mounted on
    /dev/mapper/my_vg-mylv   2002684     6168  1875072    1%   /mnt

Verification

  • View the created RAID0 stripped logical volume:

    # lvs -a -o +devices,segtype my_vg
      LV VG Attr LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices Type
      mylv my_vg rwi-a-r--- 2.00g mylv_rimage_0(0),mylv_rimage_1(0),mylv_rimage_2(0) raid0
      [mylv_rimage_0] my_vg iwi-aor--- 684.00m /dev/sdf1(0) linear
      [mylv_rimage_1] my_vg iwi-aor--- 684.00m /dev/sdg1(0) linear
      [mylv_rimage_2] my_vg iwi-aor--- 684.00m /dev/sdh1(0) linear

8.6. Configuring a stripe size for RAID LVM volumes by using the storage RHEL system role

With the storage system role, you can configure a stripe size for RAID LVM volumes on RHEL by using Red Hat Ansible Automation Platform. You can set up an Ansible playbook with the available parameters to configure an LVM pool with RAID.

Prerequisites

Procedure

  1. Create a playbook file, for example ~/playbook.yml, with the following content:

    ---
    - name: Configure stripe size for RAID LVM volumes
      hosts: managed-node-01.example.com
      roles:
        - rhel-system-roles.storage
      vars:
        storage_safe_mode: false
        storage_pools:
          - name: my_pool
            type: lvm
            disks: [sdh, sdi]
            volumes:
              - name: my_volume
                size: "1 GiB"
                mount_point: "/mnt/app/shared"
                fs_type: xfs
                raid_level: raid1
                raid_stripe_size: "256 KiB"
                state: present
  2. Validate the playbook syntax:

    $ ansible-playbook --syntax-check ~/playbook.yml

    Note that this command only validates the syntax and does not protect against a wrong but valid configuration.

  3. Run the playbook:

    $ ansible-playbook ~/playbook.yml

Additional resources

  • /usr/share/ansible/roles/rhel-system-roles.storage/README.md file
  • /usr/share/doc/rhel-system-roles/storage/ directory
  • Managing RAID

8.7. Parameters for creating a RAID0

You can create a RAID0 striped logical volume using the lvcreate --type raid0[meta] --stripes _Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath] command.

The following table describes different parameters, which you can use while creating a RAID0 striped logical volume.

Table 8.2. Parameters for creating a RAID0 striped logical volume
ParameterDescription

--type raid0[_meta]

Specifying raid0 creates a RAID0 volume without metadata volumes. Specifying raid0_meta creates a RAID0 volume with metadata volumes. Since RAID0 is non-resilient, it does not store any mirrored data blocks as RAID1/10 or calculate and store any parity blocks as RAID4/5/6 do. Hence, it does not need metadata volumes to keep state about resynchronization progress of mirrored or parity blocks. Metadata volumes become mandatory on a conversion from RAID0 to RAID4/5/6/10. Specifying raid0_meta preallocates those metadata volumes to prevent a respective allocation failure.

--stripes Stripes

Specifies the number of devices to spread the logical volume across.

--stripesize StripeSize

Specifies the size of each stripe in kilobytes. This is the amount of data that is written to one device before moving to the next device.

VolumeGroup

Specifies the volume group to use.

PhysicalVolumePath

Specifies the devices to use. If this is not specified, LVM will choose the number of devices specified by the Stripes option, one for each stripe.

8.8. Soft data corruption

Soft corruption in data storage implies that the data retrieved from a storage device is different from the data written to that device. The corrupted data can exist indefinitely on storage devices. You might not discover this corrupted data until you retrieve and attempt to use this data.

Depending on the type of configuration, a Redundant Array of Independent Disks (RAID) logical volume(LV) prevents data loss when a device fails. If a device consisting of a RAID array fails, the data can be recovered from other devices that are part of that RAID LV. However, a RAID configuration does not ensure the integrity of the data itself. Soft corruption, silent corruption, soft errors, and silent errors are terms that describe data that has become corrupted, even if the system design and software continues to function as expected.

Device mapper (DM) integrity is used with RAID levels 1, 4, 5, 6, and 10 to mitigate or prevent data loss due to soft corruption. The RAID layer ensures that a non-corrupted copy of the data can fix the soft corruption errors. The integrity layer sits above each RAID image while an extra sub LV stores the integrity metadata or data checksums for each RAID image. When you retrieve data from an RAID LV with integrity, the integrity data checksums analyze the data for corruption. If corruption is detected, the integrity layer returns an error message, and the RAID layer retrieves a non-corrupted copy of the data from another RAID image. The RAID layer automatically rewrites non-corrupted data over the corrupted data to repair the soft corruption.

When creating a new RAID LV with DM integrity or adding integrity to an existing RAID LV, consider the following points:

  • The integrity metadata requires additional storage space. For each RAID image, every 500MB data requires 4MB of additional storage space because of the checksums that get added to the data.
  • While some RAID configurations are impacted more than others, adding DM integrity impacts performance due to latency when accessing the data. A RAID1 configuration typically offers better performance than RAID5 or its variants.
  • The RAID integrity block size also impacts performance. Configuring a larger RAID integrity block size offers better performance. However, a smaller RAID integrity block size offers greater backward compatibility.
  • There are two integrity modes available: bitmap or journal. The bitmap integrity mode typically offers better performance than journal mode.
Tip

If you experience performance issues, either use RAID1 with integrity or test the performance of a particular RAID configuration to ensure that it meets your requirements.

8.9. Creating a RAID LV with DM integrity

When you create a RAID LV with device mapper (DM) integrity or add integrity to an existing RAID LV, it mitigates the risk of losing data due to soft corruption. Wait for the integrity synchronization and the RAID metadata to complete before using the LV. Otherwise, the background initialization might impact the LV’s performance.

Procedure

  1. Create a RAID LV with DM integrity. The following example creates a new RAID LV with integrity named test-lv in the my_vg volume group, with a usable size of 256M and RAID level 1:

    # lvcreate --type raid1 --raidintegrity y -L 256M -n test-lv my_vg
    Creating integrity metadata LV test-lv_rimage_0_imeta with size 8.00 MiB.
    Logical volume "test-lv_rimage_0_imeta" created.
    Creating integrity metadata LV test-lv_rimage_1_imeta with size 8.00 MiB.
    Logical volume "test-lv_rimage_1_imeta" created.
    Logical volume "test-lv" created.
    Note

    To add DM integrity to an existing RAID LV, use the following command:

    # lvconvert --raidintegrity y my_vg/test-lv

    Adding integrity to a RAID LV limits the number of operations that you can perform on that RAID LV.

  2. Optional: Remove the integrity before performing certain operations.

    # lvconvert --raidintegrity n my_vg/test-lv
    Logical volume my_vg/test-lv has removed integrity.

Verification

  • View information about the added DM integrity:

    • View information about the test-lv RAID LV that was created in the my_vg volume group:

      # lvs -a my_vg
        LV                        VG      Attr       LSize   Origin                 Cpy%Sync
        test-lv                   my_vg rwi-a-r--- 256.00m                          2.10
        [test-lv_rimage_0]        my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig] 93.75
        [test-lv_rimage_0_imeta]  my_vg ewi-ao----   8.00m
        [test-lv_rimage_0_iorig]  my_vg -wi-ao---- 256.00m
        [test-lv_rimage_1]        my_vg gwi-aor--- 256.00m [test-lv_rimage_1_iorig] 85.94
       [...]

      The following describes different options from this output:

      g attribute
      It is the list of attributes under the Attr column indicates that the RAID image is using integrity. The integrity stores the checksums in the _imeta RAID LV.
      Cpy%Sync column
      It indicates the synchronization progress for both the top level RAID LV and for each RAID image.
      RAID image
      It is is indicated in the LV column by raid_image_N.
      LV column
      It ensures that the synchronization progress displays 100% for the top level RAID LV and for each RAID image.
    • Display the type for each RAID LV:

      # lvs -a my-vg -o+segtype
        LV                       VG      Attr       LSize   Origin                 Cpy%Sync Type
        test-lv                  my_vg rwi-a-r--- 256.00m                          87.96    raid1
        [test-lv_rimage_0]       my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig] 100.00   integrity
        [test-lv_rimage_0_imeta] my_vg ewi-ao----   8.00m                                   linear
        [test-lv_rimage_0_iorig] my_vg -wi-ao---- 256.00m                                   linear
        [test-lv_rimage_1]       my_vg gwi-aor--- 256.00m [test-lv_rimage_1_iorig] 100.00   integrity
       [...]
    • There is an incremental counter that counts the number of mismatches detected on each RAID image. View the data mismatches detected by integrity from rimage_0 under my_vg/test-lv:

      # lvs -o+integritymismatches my_vg/test-lv_rimage_0
        LV                 VG      Attr       LSize   Origin                    Cpy%Sync IntegMismatches
        [test-lv_rimage_0] my_vg gwi-aor--- 256.00m [test-lv_rimage_0_iorig]    100.00                 0

      In this example, the integrity has not detected any data mismatches and thus the IntegMismatches counter shows zero (0).

    • View the data integrity information in the /var/log/messages log files, as shown in the following examples:

      Example 8.1. Example of dm-integrity mismatches from the kernel message logs

      device-mapper: integrity: dm-12: Checksum failed at sector 0x24e7

      Example 8.2. Example of dm-integrity data corrections from the kernel message logs

      md/raid1:mdX: read error corrected (8 sectors at 9448 on dm-16)

Additional resources

  • lvcreate(8) and lvmraid(7) man pages on your system

8.10. Minimum and maximum I/O rate options

When you create a RAID logical volumes, the background I/O required to initialize the logical volumes with the sync operation can expel other I/O operations to LVM devices, such as updates to volume group metadata, particularly when you are creating many RAID logical volumes. This can cause the other LVM operations to slow down.

You can control the rate at which a RAID logical volume is initialized by implementing recovery throttling. To control the rate at which sync operations are performed, set the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvcreate command.

You can specify these options as follows:

--maxrecoveryrate Rate[bBsSkKmMgG]
Sets the maximum recovery rate for a RAID logical volume so that it will not expel nominal I/O operations. Specify the Rate as an amount per second for each device in the array. If you do not provide a suffix, then it assumes kiB/sec/device. Setting the recovery rate to 0 means it will be unbounded.
--minrecoveryrate Rate[bBsSkKmMgG]
Sets the minimum recovery rate for a RAID logical volume to ensure that I/O for sync operations achieves a minimum throughput, even when heavy nominal I/O is present. Specify the Rate as an amount per second for each device in the array. If you do not give a suffix, then it assumes kiB/sec/device.

For example, use the lvcreate --type raid10 -i 2 -m 1 -L 10G --maxrecoveryrate 128 -n my_lv my_vg command to create a 2-way RAID10 array my_lv, which is in the volume group my_vg with 3 stripes that is 10G in size with a maximum recovery rate of 128 kiB/sec/device. You can also specify minimum and maximum recovery rates for a RAID scrubbing operation.

8.11. Converting a linear device to a RAID logical volume

You can convert an existing linear logical volume to a RAID logical volume. To perform this operation, use the --type argument of the lvconvert command.

RAID logical volumes are composed of metadata and data subvolume pairs. When you convert a linear device to a RAID1 array, it creates a new metadata subvolume and associates it with the original logical volume on one of the same physical volumes that the linear volume is on. The additional images are added in a metadata/data subvolume pair. If the metadata image that pairs with the original logical volume cannot be placed on the same physical volume, the lvconvert fails.

Procedure

  1. View the logical volume device that needs to be converted:

    # lvs -a -o name,copy_percent,devices my_vg
      LV     Copy%  Devices
      my_lv         /dev/sde1(0)
  2. Convert the linear logical volume to a RAID device. The following command converts the linear logical volume my_lv in volume group __my_vg, to a 2-way RAID1 array:

    # lvconvert --type raid1 -m 1 my_vg/my_lv
      Are you sure you want to convert linear LV my_vg/my_lv to raid1 with 2 images enhancing resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.

Verification

  • Ensure if the logical volume is converted to a RAID device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            6.25   my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(0)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(256)
      [my_lv_rmeta_1]         /dev/sdf1(0)

Additional resources

  • lvconvert(8) man page on your system

8.12. Converting an LVM RAID1 logical volume to an LVM linear logical volume

You can convert an existing RAID1 LVM logical volume to an LVM linear logical volume. To perform this operation, use the lvconvert command and specify the -m0 argument. This removes all the RAID data subvolumes and all the RAID metadata subvolumes that make up the RAID array, leaving the top-level RAID1 image as the linear logical volume.

Procedure

  1. Display an existing LVM RAID1 logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(1)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(0)
      [my_lv_rmeta_1]         /dev/sdf1(0)
  2. Convert an existing RAID1 LVM logical volume to an LVM linear logical volume. The following command converts the LVM RAID1 logical volume my_vg/my_lv to an LVM linear device:

    # lvconvert -m0 my_vg/my_lv
      Are you sure you want to convert raid1 LV my_vg/my_lv to type linear losing all resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.

    When you convert an LVM RAID1 logical volume to an LVM linear volume, you can also specify which physical volumes to remove. In the following example, the lvconvert command specifies that you want to remove /dev/sde1, leaving /dev/sdf1 as the physical volume that makes up the linear device:

    # lvconvert -m0 my_vg/my_lv /dev/sde1

Verification

  • Verify if the RAID1 logical volume was converted to an LVM linear device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV    Copy%  Devices
      my_lv        /dev/sdf1(1)

Additional resources

  • lvconvert(8) man page on your system

8.13. Converting a mirrored LVM device to a RAID1 logical volume

You can convert an existing mirrored LVM device with a segment type mirror to a RAID1 LVM device. To perform this operation, use the lvconvert command with the --type raid1 argument. This renames the mirror subvolumes named mimage to RAID subvolumes named rimage.

In addition, it also removes the mirror log and creates metadata subvolumes named rmeta for the data subvolumes on the same physical volumes as the corresponding data subvolumes.

Procedure

  1. View the layout of a mirrored logical volume my_vg/my_lv:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             15.20 my_lv_mimage_0(0),my_lv_mimage_1(0)
      [my_lv_mimage_0]        /dev/sde1(0)
      [my_lv_mimage_1]        /dev/sdf1(0)
      [my_lv_mlog]            /dev/sdd1(0)
  2. Convert the mirrored logical volume my_vg/my_lv to a RAID1 logical volume:

    # lvconvert --type raid1 my_vg/my_lv
    Are you sure you want to convert mirror LV my_vg/my_lv to raid1 type? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.

Verification

  • Verify if the mirrored logical volume is converted to a RAID1 logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(0)
      [my_lv_rimage_1]        /dev/sdf1(0)
      [my_lv_rmeta_0]         /dev/sde1(125)
      [my_lv_rmeta_1]         /dev/sdf1(125)

Additional resources

  • lvconvert(8) man page on your system

8.14. Commands to resize a RAID logical volume

You can resize a RAID logical volume in the following ways:

  • You can increase the size of a RAID logical volume of any type with the lvresize or lvextend command. This does not change the number of RAID images. For striped RAID logical volumes, the same stripe rounding constraints apply when you create a striped RAID logical volume.
  • You can reduce the size of a RAID logical volume of any type with the lvresize or lvreduce command. This does not change the number of RAID images. As with the lvextend command, the same stripe rounding constraints apply when you create a striped RAID logical volume.
  • You can change the number of stripes on a striped RAID logical volume such as RAID4, RAID5, RAID6, or RAID10 with the --stripes N parameter of the lvconvert command. This increases or reduces the size of the RAID logical volume by the capacity of the stripes added or removed. Note that raid10 volumes are capable only of adding stripes. This capability is part of the RAID reshaping feature and with this feature, you can change attributes of a RAID logical volume while keeping the same RAID level.

8.15. Changing the number of images in an existing RAID1 device

You can change the number of images in an existing RAID1 array, similar to the way you can change the number of images in the implementation of LVM mirroring.

When you add images to a RAID1 logical volume with the lvconvert command, you can perform the following operations:

  • specify the total number of images for the resulting device,
  • how many images to add to the device, and
  • can optionally specify on which physical volumes the new metadata/data image pairs reside.

Procedure

  1. Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV                Copy%  Devices
      my_lv             6.25    my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]         /dev/sde1(0)
      [my_lv_rimage_1]         /dev/sdf1(1)
      [my_lv_rmeta_0]          /dev/sde1(256)
      [my_lv_rmeta_1]          /dev/sdf1(0)

    Metadata subvolumes named rmeta always exist on the same physical devices as their data subvolume counterparts rimage. The metadata/data subvolume pairs will not be created on the same physical volumes as those from another metadata/data subvolume pair in the RAID array unless you specify --alloc anywhere.

  2. Convert the 2-way RAID1 logical volume my_vg/my_lv to a 3-way RAID1 logical volume:

    # lvconvert -m 2 my_vg/my_lv
    Are you sure you want to convert raid1 LV my_vg/my_lv to 3 images enhancing resilience? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.

    The following are a few examples of changing the number of images in an existing RAID1 device:

    • You can also specify which physical volumes to use while adding an image to RAID. The following command converts the 2-way RAID1 logical volume my_vg/my_lv to a 3-way RAID1 logical volume by specifying the physical volume /dev/sdd1 to use for the array:

      # lvconvert -m 2 my_vg/my_lv /dev/sdd1
    • Convert the 3-way RAID1 logical volume into a 2-way RAID1 logical volume:

      # lvconvert -m1 my_vg/my_lv
      Are you sure you want to convert raid1 LV my_vg/my_lv to 2 images reducing resilience? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.
    • Convert the 3-way RAID1 logical volume into a 2-way RAID1 logical volume by specifying the physical volume /dev/sde1, which contains the image to remove:

      # lvconvert -m1 my_vg/my_lv /dev/sde1

      Additionally, when you remove an image and its associated metadata subvolume volume, any higher-numbered images will be shifted down to fill the slot. Removing lv_rimage_1 from a 3-way RAID1 array that consists of lv_rimage_0, lv_rimage_1, and lv_rimage_2 results in a RAID1 array that consists of lv_rimage_0 and lv_rimage_1. The subvolume lv_rimage_2 will be renamed and take over the empty slot, becoming lv_rimage_1.

Verification

  • View the RAID1 device after changing the number of images in an existing RAID1 device:

    # lvs -a -o name,copy_percent,devices my_vg
      LV Cpy%Sync Devices
      my_lv 100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0] /dev/sdd1(1)
      [my_lv_rimage_1] /dev/sde1(1)
      [my_lv_rimage_2] /dev/sdf1(1)
      [my_lv_rmeta_0] /dev/sdd1(0)
      [my_lv_rmeta_1] /dev/sde1(0)
      [my_lv_rmeta_2] /dev/sdf1(0)

Additional resources

  • lvconvert(8) man page on your system

8.16. Splitting off a RAID image as a separate logical volume

You can split off an image of a RAID logical volume to form a new logical volume. When you are removing a RAID image from an existing RAID1 logical volume or removing a RAID data subvolume and its associated metadata subvolume from the middle of the device, any higher numbered images will be shifted down to fill the slot. The index numbers on the logical volumes that make up a RAID array will thus be an unbroken sequence of integers.

Note

You cannot split off a RAID image if the RAID1 array is not yet in sync.

Procedure

  1. Display the LVM device my_vg/my_lv, which is a 2-way RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             12.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sde1(1)
      [my_lv_rimage_1]        /dev/sdf1(1)
      [my_lv_rmeta_0]         /dev/sde1(0)
      [my_lv_rmeta_1]         /dev/sdf1(0)
  2. Split the RAID image into a separate logical volume:

    • The following example splits a 2-way RAID1 logical volume, my_lv, into two linear logical volumes, my_lv and new:

      # lvconvert --splitmirror 1 -n new my_vg/my_lv
      Are you sure you want to split raid1 LV my_vg/my_lv losing all resilience? [y/n]: y
    • The following example splits a 3-way RAID1 logical volume, my_lv, into a 2-way RAID1 logical volume, my_lv, and a linear logical volume, new:

      # lvconvert --splitmirror 1 -n new my_vg/my_lv

Verification

  • View the logical volume after you split off an image of a RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV      Copy%  Devices
      my_lv          /dev/sde1(1)
      new            /dev/sdf1(1)

Additional resources

  • lvconvert(8) man page on your system

8.17. Splitting and merging a RAID Image

You can temporarily split off an image of a RAID1 array for read-only use while tracking any changes by using the --trackchanges argument with the --splitmirrors argument of the lvconvert command. Using this feature, you can merge the image into an array at a later time while resyncing only those portions of the array that have changed since the image was split.

When you split off a RAID image with the --trackchanges argument, you can specify which image to split but you cannot change the name of the volume being split. In addition, the resulting volumes have the following constraints:

  • The new volume you create is read-only.
  • You cannot resize the new volume.
  • You cannot rename the remaining array.
  • You cannot resize the remaining array.
  • You can activate the new volume and the remaining array independently.

You can merge an image that was split off. When you merge the image, only the portions of the array that have changed since the image was split are resynced.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
      Logical volume "my_lv" created
  2. Optional: View the created RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv          100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  3. Split an image from the created RAID logical volume and track the changes to the remaining array:

    # lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
      my_lv_rimage_2 split from my_lv for read-only purposes.
      Use 'lvconvert --merge my_vg/my_lv_rimage_2' to merge back into my_lv
  4. Optional: View the logical volume after splitting the image:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sdc1(1)
      [my_lv_rimage_1]          /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdc1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
  5. Merge the volume back into the array:

    # lvconvert --merge my_vg/my_lv_rimage_1
      my_vg/my_lv_rimage_1 successfully merged back into my_vg/my_lv

Verification

  • View the merged logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sdc1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdc1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)

Additional resources

  • lvconvert(8) man page on your system

8.18. Setting a RAID fault policy

Based on the raid_fault_policy field preferences in the /etc/lvm/lvm.conf file, LVM RAID automatically handles device failures. You can set raid_fault_policy field to any one of the following parameter depending on the requirement:

warn

You can this parameter to manually repair the failed device and display warnings by using system logs.

By default, the value of the raid_fault_policy field is warn in lvm.conf. If enough devices are operational, the RAID logical volume continues to operate.

allocate
You can use this parameter to automatically replace the failed device.

8.18.1. Setting the RAID fault policy to allocate

You can set the raid_fault_policy field to the allocate parameter in the /etc/lvm/lvm.conf file. With this preference, the system attempts to replace the failed device with a spare device from the volume group. If there is no spare device, the system log includes this information.

Procedure

  1. View the RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
    
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  2. View the RAID logical volume if the /dev/sdb device fails:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdb: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        [unknown](1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [...]

    You can also view the system log for the error messages if the /dev/sdb device fails.

  3. Set the raid_fault_policy field to allocate in the lvm.conf file:

     # vi /etc/lvm/lvm.conf
     raid_fault_policy = "allocate"
    Note

    If you set raid_fault_policy to allocate but there are no spare devices, the allocation fails, leaving the logical volume as it is. If the allocation fails, you can fix and replace the failed device by using the lvconvert --repair command. For more information, see Replacing a failed RAID device in a logical volume.

Verification

  • Verify if the failed device is now replaced with a new device from the volume group:

    # lvs -a -o name,copy_percent,devices my_vg
      Couldn't find device with uuid 3lugiV-3eSP-AFAR-sdrP-H20O-wM2M-qdMANy.
      LV            Copy%  Devices
      lv            100.00 lv_rimage_0(0),lv_rimage_1(0),lv_rimage_2(0)
      [lv_rimage_0]        /dev/sdh1(1)
      [lv_rimage_1]        /dev/sdc1(1)
      [lv_rimage_2]        /dev/sdd1(1)
      [lv_rmeta_0]         /dev/sdh1(0)
      [lv_rmeta_1]         /dev/sdc1(0)
      [lv_rmeta_2]         /dev/sdd1(0)
    Note

    Even though the failed device is now replaced, the display still indicates that LVM could not find the failed device because the device is not yet removed from the volume group. You can remove the failed device from the volume group by executing the vgreduce --removemissing my_vg command.

Additional resources

  • lvm.conf(5) man page on your system

8.18.2. Setting the RAID fault policy to warn

You can set the raid_fault_policy field to the warn parameter in the lvm.conf file. With this preference, the system adds a warning to the system log that indicates a failed device. Based on the warning, you can determine the further steps.

Procedure

  1. View the RAID logical volume:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc1(1)
      [my_lv_rimage_2]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc1(0)
      [my_lv_rmeta_2]         /dev/sdd1(0)
  2. Set the raid_fault_policy field to warn in the lvm.conf file:

    # vi /etc/lvm/lvm.conf
     # This configuration option has an automatic default value.
     raid_fault_policy = "warn"
  3. View the system log to display error messages if the /dev/sdb device fails:

    # grep lvm /var/log/messages
    
    Apr 14 18:48:59 virt-506 kernel: sd 25:0:0:0: rejecting I/O to offline device
    Apr 14 18:48:59 virt-506 kernel: I/O error, dev sdb, sector 8200 op 0x1:(WRITE) flags 0x20800 phys_seg 0 prio class 2
    [...]
    Apr 14 18:48:59 virt-506 dmeventd[91060]: WARNING: VG my_vg is missing PV 9R2TVV-bwfn-Bdyj-Gucu-1p4F-qJ2Q-82kCAF (last written to /dev/sdb).
    Apr 14 18:48:59 virt-506 dmeventd[91060]: WARNING: Couldn't find device with uuid 9R2TVV-bwfn-Bdyj-Gucu-1p4F-qJ2Q-82kCAF.
    Apr 14 18:48:59 virt-506 dmeventd[91060]: Use 'lvconvert --repair my_vg/ly_lv' to replace failed device.

    If the /dev/sdb device fails, the system log displays error messages. In this case, however, LVM will not automatically attempt to repair the RAID device by replacing one of the images. Instead, if the device has failed you can replace the device with the --repair argument of the lvconvert command. For more information, see Replacing a failed RAID device in a logical volume.

Additional resources

  • lvm.conf(5) man page on your system

8.19. Replacing a RAID device in a logical volume

You can replace a RAID device in a logical volume depending on the following scenarios:

  • Replacing a working RAID device.
  • Replacing a failed RAID device in a logical volume.

8.19.1. Replacing a working RAID device

You can replace a working RAID device in a logical volume by using the --replace argument of the lvconvert command.

Warning

In the case of RAID device failure, the following commands do not work.

Prerequisites

  • The RAID device has not failed.

Procedure

  1. Create a RAID1 array:

    # lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
      Logical volume "my_lv" created
  2. Examine the created RAID1 array:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdb2(1)
      [my_lv_rimage_2]        /dev/sdc1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdb2(0)
      [my_lv_rmeta_2]         /dev/sdc1(0)
  3. Replace the RAID device with any of the following methods depending on your requirements:

    1. Replace a RAID1 device by specifying the physical volume that you want to replace:

      # lvconvert --replace /dev/sdb2 my_vg/my_lv
    2. Replace a RAID1 device by specifying the physical volume to use for the replacement:

      # lvconvert --replace /dev/sdb1 my_vg/my_lv /dev/sdd1
    3. Replace multiple RAID devices at a time by specifying multiple replace arguments:

      # lvconvert --replace /dev/sdb1 --replace /dev/sdc1 my_vg/my_lv

Verification

  1. Examine the RAID1 array after specifying the physical volume that you wanted to replace:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             37.50 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sdb1(1)
      [my_lv_rimage_1]        /dev/sdc2(1)
      [my_lv_rimage_2]        /dev/sdc1(1)
      [my_lv_rmeta_0]         /dev/sdb1(0)
      [my_lv_rmeta_1]         /dev/sdc2(0)
      [my_lv_rmeta_2]         /dev/sdc1(0)
  2. Examine the RAID1 array after specifying the physical volume to use for the replacement:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             28.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
      [my_lv_rimage_0]        /dev/sda1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rmeta_0]         /dev/sda1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
  3. Examine the RAID1 array after replacing multiple RAID devices at a time:

    # lvs -a -o name,copy_percent,devices my_vg
      LV               Copy%  Devices
      my_lv             60.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]        /dev/sda1(1)
      [my_lv_rimage_1]        /dev/sdd1(1)
      [my_lv_rimage_2]        /dev/sde1(1)
      [my_lv_rmeta_0]         /dev/sda1(0)
      [my_lv_rmeta_1]         /dev/sdd1(0)
      [my_lv_rmeta_2]         /dev/sde1(0)

Additional resources

  • lvconvert(8) man page on your system

8.19.2. Replacing a failed RAID device in a logical volume

RAID is not similar to traditional LVM mirroring. In case of LVM mirroring, remove the failed devices. Otherwise, the mirrored logical volume would hang while RAID arrays continue running with failed devices. For RAID levels other than RAID1, removing a device would mean converting to a lower RAID level, for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0.

Instead of removing a failed device and allocating a replacement, with LVM, you can replace a failed device that serves as a physical volume in a RAID logical volume by using the --repair argument of the lvconvert command.

Prerequisites

  • The volume group includes a physical volume that provides enough free capacity to replace the failed device.

    If no physical volume with enough free extents is available on the volume group, add a new, sufficiently large physical volume by using the vgextend utility.

Procedure

  1. View the RAID logical volume:

    # lvs --all --options name,copy_percent,devices my_vg
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdc1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdc1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  2. View the RAID logical volume after the /dev/sdc device fails:

    # lvs --all --options name,copy_percent,devices my_vg
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          [unknown](1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           [unknown](0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  3. Replace the failed device:

    # lvconvert --repair my_vg/my_lv
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
    Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
    Faulty devices in my_vg/my_lv successfully replaced.
  4. Optional: Manually specify the physical volume that replaces the failed device:

    # lvconvert --repair my_vg/my_lv replacement_pv
  5. Examine the logical volume with the replacement:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdc: open failed: No such device or address
      /dev/sdc1: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      LV               Cpy%Sync Devices
      my_lv            43.79    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

    Until you remove the failed device from the volume group, LVM utilities still indicate that LVM cannot find the failed device.

  6. Remove the failed device from the volume group:

    # vgreduce --removemissing my_vg

Verification

  1. View the available physical volumes after removing the failed device:

    # pvscan
    PV /dev/sde1 VG rhel_virt-506 lvm2 [<7.00 GiB / 0 free]
    PV /dev/sdb1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
  2. Examine the logical volume after the replacing the failed device:

    # lvs --all --options name,copy_percent,devices my_vg
    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

Additional resources

  • lvconvert(8) and vgreduce(8) man pages on your system

8.20. Checking data coherency in a RAID logical volume

LVM provides scrubbing support for RAID logical volumes. RAID scrubbing is the process of reading all the data and parity blocks in an array and checking to see whether they are coherent. The lvchange --syncaction repair command initiates a background synchronization action on the array. The following attributes provide details about data coherency:

  • The raid_sync_action field displays the current synchronization action that the RAID logical volume is performing. It can be one of the following values:

    idle
    Completed all sync actions (doing nothing).
    resync
    Initializing or resynchronizing an array after an unclean machine shutdown.
    recover
    Replacing a device in the array.
    check
    Looking for array inconsistencies.
    repair
    Looking for and repairing inconsistencies.
  • The raid_mismatch_count field displays the number of discrepancies found during a check action.
  • The Cpy%Sync field displays the progress of the sync actions.
  • The lv_attr field provides additional indicators. Bit 9 of this field displays the health of the logical volume, and it supports the following indicators:

    m or mismatches
    Indicates that there are discrepancies in a RAID logical volume. You can see this character after the scrubbing operation detects the portions of the RAID, which are not coherent.
    r or refresh
    Indicates a failed device in a RAID array, even though LVM can read the device label and considers the device to be operational. Refresh the logical volume to notify the kernel that the device is now available, or replace the device if you suspect that it failed.

Procedure

  1. Optional: Limit the I/O bandwidth that the scrubbing process uses. When you perform a RAID scrubbing operation, the background I/O required by the sync actions can crowd out other I/O to LVM devices, such as updates to volume group metadata. This might cause the other LVM operations to slow down.

    You can control the rate of the scrubbing operation by implementing recovery throttling. You can set the recovery rate using --maxrecoveryrate Rate[bBsSkKmMgG] or --minrecoveryrate Rate[bBsSkKmMgG] with the lvchange --syncaction commands. For more information, see Minimum and maximum I/O rate options.

    Specify the Rate value as an amount per second for each device in the array. If you provide no suffix, the options assume kiB per second per device.

  2. Display the number of discrepancies in the array, without repairing them:

    # lvchange --syncaction check my_vg/my_lv

    This command initiates a background synchronization action on the array.

  3. Optional: View the var/log/syslog file for the kernel messages.
  4. Correct the discrepancies in the array:

    # lvchange --syncaction repair my_vg/my_lv

    This command repairs or replaces failed devices in a RAID logical volume. You can view the var/log/syslog file for the kernel messages after executing this command.

Verification

  1. Display information about the scrubbing operation:

    # lvs -o +raid_sync_action,raid_mismatch_count my_vg/my_lv
    LV    VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert SyncAction Mismatches
    my_lv my_vg rwi-a-r--- 500.00m                                    100.00           idle        0

Additional resources

8.21. Converting a RAID logical volume to another RAID level

LVM supports RAID takeover, which means converting a RAID logical volume from one RAID level to another, for example, from RAID 5 to RAID 6. You can change the RAID level to increase or decrease resilience to device failures.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid5 -i 3 -L 500M -n my_lv my_vg
    Using default stripesize 64.00 KiB.
    Rounding size 500.00 MiB (125 extents) up to stripe boundary size 504.00 MiB (126 extents).
    Logical volume "my_lv" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices,segtype
      LV               VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices                                                                 Type
      my_lv            my_vg         rwi-a-r--- 504.00m                                    100.00           my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0),my_lv_rimage_3(0) raid5
      [my_lv_rimage_0] my_vg         iwi-aor--- 168.00m                                                     /dev/sda(1)                                                             linear
  3. Convert the RAID logical volume to another RAID level:

    # lvconvert --type raid6 my_vg/my_lv
    Using default stripesize 64.00 KiB.
    Replaced LV type raid6 (same as raid6_zr) with possible type raid6_ls_6.
    Repeat this command to convert to raid6 after an interim conversion has finished.
    Are you sure you want to convert raid5 LV my_vg/my_lv to raid6_ls_6 type? [y/n]: y
    Logical volume my_vg/my_lv successfully converted.
  4. Optional: If this command prompts to repeat the conversion, run:

    # lvconvert --type raid6 my_vg/my_lv

Verification

  1. View the RAID logical volume with the converted RAID level:

    # lvs -a -o +devices,segtype
      LV               VG            Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices                                                                                   Type
      my_lv            my_vg         rwi-a-r--- 504.00m                                    100.00           my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0),my_lv_rimage_3(0),my_lv_rimage_4(0) raid6
      [my_lv_rimage_0] my_vg         iwi-aor--- 172.00m                                                     /dev/sda(1)                                                                               linear

Additional resources

  • lvconvert(8) and lvmraid(8) man pages on your system

8.22. I/O Operations on a RAID1 logical volume

You can control the I/O operations for a device in a RAID1 logical volume by using the --writemostly and --writebehind parameters of the lvchange command. The following is the format for using these parameters:

--[raid]writemostly PhysicalVolume[:{t|y|n}]

Marks a device in a RAID1 logical volume as write-mostly and avoids all read actions to these drives unless necessary. Setting this parameter keeps the number of I/O operations to the drive to a minimum. Use the lvchange --writemostly /dev/sdb my_vg/ly_lv command to set this parameter.

You can set the writemostly attribute in the following ways:

:y
By default, the value of the writemostly attribute is yes for the specified physical volume in the logical volume.
:n
To remove the writemostly flag, append :n to the physical volume.
:t
To toggle the value of the writemostly attribute, specify the --writemostly argument. You can use this argument more than one time in a single command, to toggle the writemostly attributes for all the physical volumes in a logical volume at once.
--[raid]writebehind IOCount

Specifies the maximum number of pending writes marked as writemostly. These are the number of write operations applicable to devices in a RAID1 logical volume. After the value of this parameter exceeds, all write actions to the constituent devices complete synchronously before the RAID array notifies for completion of all write actions.

You can set this parameter by using the lvchange --writebehind 100 my_vg/ly_lv command. Setting the writemostly attribute’s value to zero clears the preference. With this setting, the system chooses the value arbitrarily.

8.23. Reshaping a RAID volume

RAID reshaping means changing attributes of a RAID logical volume without changing the RAID level. Some attributes that you can change include RAID layout, stripe size, and number of stripes.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid5 -i 2 -L 500M -n my_lv my_vg
    
    Using default stripesize 64.00 KiB.
    Rounding size 500.00 MiB (125 extents) up to stripe boundary size 504.00 MiB (126 extents).
    Logical volume "my_lv" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices
    
    LV               VG    Attr       LSize   Pool   Origin Data% Meta% Move Log Cpy%Sync Convert Devices
    my_lv            my_vg rwi-a-r--- 504.00m                                    100.00            my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
    [my_lv_rimage_0] my_vg iwi-aor--- 252.00m                                                      /dev/sda(1)
    [my_lv_rimage_1] my_vg iwi-aor--- 252.00m                                                      /dev/sdb(1)
    [my_lv_rimage_2] my_vg iwi-aor--- 252.00m                                                      /dev/sdc(1)
    [my_lv_rmeta_0]  my_vg ewi-aor---   4.00m                                                      /dev/sda(0)
    [my_lv_rmeta_1]  my_vg ewi-aor---   4.00m                                                      /dev/sdb(0)
    [my_lv_rmeta_2]  my_vg ewi-aor---   4.00m                                                      /dev/sdc(0)
  3. Optional: View the stripes images and stripesize of the RAID logical volume:

    # lvs -o stripes my_vg/my_lv
      #Str
         3
    # lvs -o stripesize my_vg/my_lv
      Stripe
      64.00k
  4. Modify the attributes of the RAID logical volume by using the following ways depending on your requirement:

    1. Modify the stripes images of the RAID logical volume:

      # lvconvert --stripes 3 my_vg/my_lv
      Using default stripesize 64.00 KiB.
      WARNING: Adding stripes to active logical volume my_vg/my_lv will grow it from 126 to 189 extents!
      Run "lvresize -l126 my_vg/my_lv" to shrink it or use the additional capacity.
      Are you sure you want to add 1 images to raid5 LV my_vg/my_lv? [y/n]: y
      Logical volume my_vg/my_lv successfully converted.
    2. Modify the stripesize of the RAID logical volume:

      # lvconvert --stripesize 128k my_vg/my_lv
        Converting stripesize 64.00 KiB of raid5 LV my_vg/my_lv to 128.00 KiB.
      Are you sure you want to convert raid5 LV my_vg/my_lv? [y/n]: y
        Logical volume my_vg/my_lv successfully converted.
    3. Modify the maxrecoveryrate and minrecoveryrate attributes:

      # lvchange --maxrecoveryrate 4M my_vg/my_lv
        Logical volume my_vg/my_lv changed.
      # lvchange --minrecoveryrate 1M my_vg/my_lv
        Logical volume my_vg/my_lv changed.
    4. Modify the syncaction attribute:

      # lvchange --syncaction check my_vg/my_lv
    5. Modify the writemostly and writebehind attributes:

      # lvchange --writemostly /dev/sdb my_vg/my_lv
        Logical volume my_vg/my_lv changed.
      # lvchange --writebehind 100 my_vg/my_lv
        Logical volume my_vg/my_lv changed.

Verification

  1. View the stripes images and stripesize of the RAID logical volume:

    # lvs -o stripes my_vg/my_lv
      #Str
         4
    # lvs -o stripesize my_vg/my_lv
      Stripe
      128.00k
  2. View the RAID logical volume after modifying the maxrecoveryrate attribute:

    # lvs -a -o +raid_max_recovery_rate
      LV               VG       Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert MaxSync
      my_lv            my_vg    rwi-a-r---  10.00g                                     100.00           4096
      [my_lv_rimage_0] my_vg    iwi-aor---  10.00g
     [...]
  3. View the RAID logical volume after modifying the minrecoveryrate attribute:

    # lvs -a -o +raid_min_recovery_rate
      LV               VG     Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert MinSync
      my_lv            my_vg  rwi-a-r---  10.00g                                     100.00           1024
      [my_lv_rimage_0] my_vg  iwi-aor---  10.00g
      [...]
  4. View the RAID logical volume after modifying the syncaction attribute:

    # lvs -a
      LV               VG      Attr        LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
      my_lv            my_vg   rwi-a-r---  10.00g                                     2.66
      [my_lv_rimage_0] my_vg   iwi-aor---  10.00g
      [...]

Additional resources

  • lvconvert(8) and lvmraid(8) man pages on your system

8.24. Changing the region size on a RAID logical volume

When you create a RAID logical volume, the raid_region_size parameter from the /etc/lvm/lvm.conf file represents the region size for the RAID logical volume. After you created a RAID logical volume, you can change the region size of the volume. This parameter defines the granularity to keep track of the dirty or clean state. Dirty bits in the bitmap define the work set to synchronize after a dirty shutdown of a RAID volume, for example, a system failure.

If you set raid_region_size to a higher value, it reduces the size of bitmap as well as the congestion. But it impacts the write operation during resynchronizing the region because writes to RAID are postponed until synchronizing the region finishes.

Procedure

  1. Create a RAID logical volume:

    # lvcreate --type raid1 -m 1 -L 10G test
      Logical volume "lvol0" created.
  2. View the RAID logical volume:

    # lvs -a -o +devices,region_size
    
    LV                VG      Attr     LSize Pool Origin Data% Meta% Move Log   Cpy%Sync Convert Devices                              Region
    lvol0             test rwi-a-r--- 10.00g                                    100.00           lvol0_rimage_0(0),lvol0_rimage_1(0)  2.00m
    [lvol0_rimage_0]  test iwi-aor--- 10.00g                                                     /dev/sde1(1)                            0
    [lvol0_rimage_1]  test iwi-aor--- 10.00g                                                     /dev/sdf1(1)                            0
    [lvol0_rmeta_0]   test ewi-aor---  4.00m                                                     /dev/sde1(0)                            0
    [lvol0_rmeta_1]   test ewi-aor---  4.00m

    The Region column indicates the raid_region_size parameter’s value.

  3. Optional: View the raid_region_size parameter’s value:

    # cat /etc/lvm/lvm.conf | grep raid_region_size
    
    # Configuration option activation/raid_region_size.
    	# raid_region_size = 2048
  4. Change the region size of a RAID logical volume:

    # lvconvert -R 4096K my_vg/my_lv
    
    Do you really want to change the region_size 512.00 KiB of LV my_vg/my_lv to 4.00 MiB? [y/n]: y
      Changed region size on RAID LV my_vg/my_lv to 4.00 MiB.
  5. Resynchronize the RAID logical volume:

    # lvchange --resync my_vg/my_lv
    
    Do you really want to deactivate logical volume my_vg/my_lv to resync it? [y/n]: y

Verification

  1. View the RAID logical volume:

    # lvs -a -o +devices,region_size
    
    LV               VG   Attr        LSize Pool Origin Data% Meta% Move Log Cpy%Sync Convert Devices                              Region
    lvol0            test rwi-a-r--- 10.00g                                    6.25           lvol0_rimage_0(0),lvol0_rimage_1(0)  4.00m
    [lvol0_rimage_0] test iwi-aor--- 10.00g                                                   /dev/sde1(1)                            0
    [lvol0_rimage_1] test iwi-aor--- 10.00g                                                   /dev/sdf1(1)                            0
    [lvol0_rmeta_0]  test ewi-aor---  4.00m                                                   /dev/sde1(0)                            0

    The Region column indicates the changed value of the raid_region_size parameter.

  2. View the raid_region_size parameter’s value in the lvm.conf file:

    # cat /etc/lvm/lvm.conf | grep raid_region_size
    
    # Configuration option activation/raid_region_size.
    	# raid_region_size = 4096

Additional resources

  • lvconvert(8) man page on your system

Chapter 9. Limiting LVM device visibility and usage

You can limit the devices that are visible and usable to Logical Volume Manager (LVM) by controlling the devices that LVM can scan.

To adjust the configuration of LVM device scanning, edit the LVM device filter settings in the /etc/lvm/lvm.conf file. The filters in the lvm.conf file consist of a series of simple regular expressions. The system applies these expressions to each device name in the /dev directory to decide whether to accept or reject each detected block device.

9.1. Persistent identifiers for LVM filtering

Traditional Linux device names, such as /dev/sda, are subject to changes during system modifications and reboots. Persistent Naming Attributes (PNAs) like World Wide Identifier (WWID), Universally Unique Identifier (UUID), and path names are based on unique characteristics of the storage devices and are resilient to changes in hardware configurations. This makes them more stable and predictable across system reboots.

Implementation of persistent device identifiers in LVM filtering enhances the stability and reliability of LVM configurations. It also reduces the risk of system boot failures associated with the dynamic nature of device names.

9.2. The LVM device filter

The Logical Volume Manager (LVM) device filter is a list of device name patterns. You can use it to specify a set of mandatory criteria by which the system can evaluate devices and consider them as valid for use with LVM. The LVM device filter enables you control over which devices LVM uses. This can help to prevent accidental data loss or unauthorized access to storage devices.

9.2.1. LVM device filter pattern characteristics

The patterns of LVM device filter are in the form of regular expression. A regular expression delimits with a character and precedes with either a for acceptance, or r for rejection. The first regular expression in the list that matches a device determines if LVM accepts or rejects (ignores) a specific device. Then, LVM looks for the initial regular expression in the list that matches the path of a device. LVM uses this regular expression to determine whether the device should be approved with an a outcome or rejected with an r outcome.

If a single device has multiple path names, LVM accesses these path names according to their order of listing. Before any r pattern, if at least one path name matches an a pattern, LVM approves the device. However, if all path names are consistent with an r pattern before an a pattern is found, the device is rejected.

Path names that do not match the pattern do not affect the approval status of the device. If no path names correspond to a pattern for a device, LVM still approves the device.

For each device on the system, the udev rules generate multiple symlinks. Directories contain symlinks, such as /dev/disk/by-id/, /dev/disk/by-uuid/, /dev/disk/by-path/ to ensure that each device on the system is accessible through multiple path names.

To reject a device in the filter, all of the path names associated with that particular device must match the corresponding reject r expressions. However, identifying all possible path names to reject can be challenging. This is why it is better to create filters that specifically accept certain paths and reject all others, using a series of specific a expressions followed by a single r|.*| expression that rejects everything else.

While defining a specific device in the filter, use a symlink name for that device instead of the kernel name. The kernel name for a device can change, such as /dev/sda while certain symlink names do not change such as /dev/disk/by-id/wwn-*.

The default device filter accepts all devices connected to the system. An ideal user configured device filter accepts one or more patterns and rejects everything else. For example, the pattern list ending with r|.*|.

You can find the LVM devices filter configuration in the devices/filter and devices/global_filter configuration fields in the lvm.conf file. The devices/filter and devices/global_filter configuration fields are equivalent.

Additional resources

  • lvm.conf(5) man page on your system

9.2.2. Examples of LVM device filter configurations

The following examples display the filter configurations to control the devices that LVM scans and uses later. To configure the device filter in the lvm.conf file, see Applying an LVM device filter configuration

Note

You might encounter duplicate Physical Volume (PV) warnings when dealing with copied or cloned PVs. You can set up filters to resolve this. See the example filter configurations in Example LVM device filters that prevent duplicate PV warnings.

  • To scan all the devices, enter:

    filter = [ "a|.*|" ]
  • To remove the cdrom device to avoid delays if the drive contains no media, enter:

    filter = [ "r|^/dev/cdrom$|" ]
  • To add all loop devices and remove all other devices, enter:

    filter = [ "a|loop|", "r|.*|" ]
  • To add all loop and SCSI devices and remove all other block devices, enter:

    filter = [ "a|loop|", "a|/dev/sd.*|", "r|.*|" ]
  • To add only partition 8 on the first SCSI drive and remove all other block devices, enter:

    filter = [ "a|^/dev/sda8$|", "r|.*|" ]
  • To add all partitions from a specific device identified by WWID along with all multipath devices, enter:

    filter = [ "a|/dev/disk/by-id/<disk-id>.|", "a|/dev/mapper/mpath.|", "r|.*|" ]

    The command also removes any other block devices.

9.2.3. Applying an LVM device filter configuration

You can control which devices LVM scans by setting up filters in the lvm.conf configuration file.

Prerequisites

  • You have prepared the device filter pattern that you want to use.

Procedure

  1. Use the following command to test the device filter pattern, without actually modifying the /etc/lvm/lvm.conf file. The following includes an example filter configuration.

    # lvs --config 'devices{ filter = [ "a|/dev/emcpower.*|", "r|*.|" ] }'
  2. Add the device filter pattern in the configuration section devices of the /etc/lvm/lvm.conf file:

    filter = [ "a|/dev/emcpower.*|", "r|*.|" ]
  3. Scan only necessary devices on reboot:

    # dracut --force --verbose

    This command rebuilds the initramfs file system so that LVM scans only the necessary devices at the time of reboot.

Chapter 10. Controlling LVM allocation

By default, a volume group uses the normal allocation policy. This allocates physical extents according to common-sense rules such as not placing parallel stripes on the same physical volume. You can specify a different allocation policy (contiguous, anywhere, or cling) by using the --alloc argument of the vgcreate command. In general, allocation policies other than normal are required only in special cases where you need to specify unusual or nonstandard extent allocation.

10.1. Allocating extents from specified devices

You can restrict the allocation from specific devices by using the device arguments at the end of the command line with the lvcreate and the lvconvert commands. You can specify the actual extent ranges for each device for more control. The command only allocates extents for the new logical volume (LV) by using the specified physical volume (PV) as arguments. It takes available extents from each PV until they run out and then takes extents from the next PV listed. If there is not enough space on all the listed PVs for the requested LV size, then command fails. Note that the command only allocates from the named PVs. Raid LVs use sequential PVs for separate raid images or separate stripes. If the PVs are not large enough for an entire raid image, then the resulting device use is not entirely predictable.

Procedure

  1. Create a volume group (VG):

    # vgcreate <vg_name> <PV> ...

    Where:

    • <vg_name> is the name of the VG.
    • <PV> are the PVs.
  2. You can allocate PV to create different volume types, such as linear or raid:

    1. Allocate extents to create a linear volume:

      # lvcreate -n <lv_name> -L <lv_size> <vg_name> [ <PV> ... ]

      Where:

      • <lv_name> is the name of the LV.
      • <lv_size> is the size of the LV. Default unit is megabytes.
      • <vg_name> is the name of the VG.
      • [ <PV …​> ] are the PVs.

        You can specify one of the PVs, all of them, or none on the command line:

        • If you specify one PV, extents for that LV will be allocated from it.

          Note

          If the PV does not have sufficient free extents for the entire LV, then the lvcreate fails.

        • If you specify two PVs, extents for that LV will be allocated from one of them, or a combination of both.
        • If you do not specify any PV, extents will be allocated from one of the PVs in the VG, or any combination of all PVs in the VG.

          Note

          In these cases, LVM might not use all of the named or available PVs. If the first PV has sufficient free extents for the entire LV, then the other PV will probably not be used. However, if the first PV does not have a set allocation size of free extents, then LV might be allocated partly from the first PV and partly from the second PV.

          Example 10.1. Allocating extents from one PV

          In this example, lv1 extents will be allocated from sda.

          # lvcreate -n lv1 -L1G vg /dev/sda

          Example 10.2. Allocating extents from two PVs

          In this example, lv2 extents will be allocated from either sda, or sdb, or a combination of both.

          # lvcreate -n lv2 L1G vg /dev/sda /dev/sdb

          Example 10.3. Allocating extents without specifying PV

          In this example, lv3 extents will be allocated from one of the PVs in the VG, or any combination of all PVs in the VG.

          # lvcreate -n lv3 -L1G vg

          or

    2. Allocate extents to create a raid volume:

      # lvcreate --type <segment_type> -m <mirror_images> -n <lv_name> -L <lv_size> <vg_name> [ <PV> ... ]

      Where:

      • <segment_type> is the specified segment type (for example raid5, mirror, snapshot).
      • <mirror_images> creates a raid1 or a mirrored LV with the specified number of images. For example, -m 1 would result in a raid1 LV with two images.
      • <lv_name> is the name of the LV.
      • <lv_size> is the size of the LV. Default unit is megabytes.
      • <vg_name> is the name of the VG.
      • <[PV …​]> are the PVs.

        The first raid image will be allocated from the first PV, the second raid image from the second PV, and so on.

        Example 10.4. Allocating raid images from two PVs

        In this example, lv4 first raid image will be allocated from sda and second image will be allocated from sdb.

        # lvcreate --type raid1 -m 1 -n lv4 -L1G vg /dev/sda /dev/sdb

        Example 10.5. Allocating raid images from three PVs

        In this example, lv5 first raid image will be allocated from sda, second image will be allocated from sdb, and third image will be allocated from sdc.

        # lvcreate --type raid1 -m 2 -n lv5 -L1G vg /dev/sda /dev/sdb /dev/sdc

Additional resources

  • lvcreate(8), lvconvert(8), and lvmraid(7) man pages on your system

10.2. LVM allocation policies

When an LVM operation must allocate physical extents for one or more logical volumes (LVs), the allocation proceeds as follows:

  • The complete set of unallocated physical extents in the volume group is generated for consideration. If you supply any ranges of physical extents at the end of the command line, only unallocated physical extents within those ranges on the specified physical volumes (PVs) are considered.
  • Each allocation policy is tried in turn, starting with the strictest policy (contiguous) and ending with the allocation policy specified using the --alloc option or set as the default for the particular LV or volume group (VG). For each policy, working from the lowest-numbered logical extent of the empty LV space that needs to be filled, as much space as possible is allocated, according to the restrictions imposed by the allocation policy. If more space is needed, LVM moves on to the next policy.

The allocation policy restrictions are as follows:

  • The contiguous policy requires that the physical location of any logical extent is adjacent to the physical location of the immediately preceding logical extent, with the exception of the first logical extent of a LV.

    When a LV is striped or mirrored, the contiguous allocation restriction is applied independently to each stripe or raid image that needs space.

  • The cling allocation policy requires that the PV used for any logical extent be added to an existing LV that is already in use by at least one logical extent earlier in that LV.
  • An allocation policy of normal will not choose a physical extent that shares the same PV as a logical extent already allocated to a parallel LV (that is, a different stripe or raid image) at the same offset within that parallel LV.
  • If there are sufficient free extents to satisfy an allocation request but a normal allocation policy would not use them, the anywhere allocation policy will, even if that reduces performance by placing two stripes on the same PV.

You can change the allocation policy by using the vgchange command.

Note

Future updates can bring code changes in layout behavior according to the defined allocation policies. For example, if you supply on the command line two empty physical volumes that have an identical number of free physical extents available for allocation, LVM currently considers using each of them in the order they are listed; there is no guarantee that future releases will maintain that property. If you need a specific layout for a particular LV, build it up through a sequence of lvcreate and lvconvert steps such that the allocation policies applied to each step leave LVM no discretion over the layout.

10.3. Preventing allocation on a physical volume

You can prevent allocation of physical extents on the free space of one or more physical volumes with the pvchange command. This might be necessary if there are disk errors, or if you will be removing the physical volume.

Procedure

  • Use the following command to disallow the allocation of physical extents on device_name:

    # pvchange -x n /dev/sdk1

    You can also allow allocation where it had previously been disallowed by using the -xy arguments of the pvchange command.

Additional resources

  • pvchange(8) man page on your system

Chapter 11. Grouping LVM objects with tags

You can assign tags to Logical Volume Manager (LVM) objects to group them. With this feature, you can automate the control of LVM behavior, such as activation, by a group. You can also use tags instead of LVM objects arguments.

11.1. LVM object tags

A Logical Volume Manager (LVM) tag groups LVM objects of the same type. You can attach tags to objects such as physical volumes, volume groups, and logical volumes , as well as to hosts in a cluster configuration .

To avoid ambiguity, prefix each tag with @. Each tag is expanded by replacing it with all the objects that possess that tag and that are of the type expected by its position on the command line.

LVM tags are strings of up to 1024 characters. LVM tags cannot start with a hyphen.

A valid tag consists of a limited range of characters only. The allowed characters are A-Z a-z 0-9 _ + . - / = ! : # &.

Only objects in a volume group can be tagged. Physical volumes lose their tags if they are removed from a volume group; this is because tags are stored as part of the volume group metadata and that is deleted when a physical volume is removed.

You can apply some commands to all volume groups (VG), logical volumes (LV), or physical volumes (PV) that have the same tag. The man page of the given command shows the syntax, such as VG|Tag, LV|Tag, or PV|Tag when you can substitute a tag name for a VG, LV, or PV name.

11.2. Listing LVM tags

The following example shows how to list LVM tags.

Procedure

  • Use the following command to list all the logical volumes with the database tag:
# lvs @database
  • Use the following command to list the currently active host tags:
# lvm tags

11.3. Adding tags to LVM objects

You can add tags to LVM objects to group them by using the --addtag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.

Procedure

  • To add a tag to an existing PV, use:

    # pvchange --addtag <@tag> <PV>
  • To add a tag to an existing VG, use:

    # vgchange --addtag <@tag> <VG>
  • To add a tag to a VG during creation, use:

    # vgcreate --addtag <@tag> <VG>
  • To add a tag to an existing LV, use:

    # lvchange --addtag <@tag> <LV>
  • To add a tag to a LV during creation, use:

    # lvcreate --addtag <@tag> ...

11.4. Removing tags from LVM objects

If you no longer want to keep your LVM objects grouped, you can remove tags from the objects by using the --deltag option with various volume management commands.

Prerequisites

  • The lvm2 package is installed.
  • You have created tags on physical volumes (PV), volume groups (VG), or logical volumes (LV).

Procedure

  • To remove a tag from an existing PV, use:

    # pvchange --deltag @tag PV
  • To remove a tag from an existing VG, use:

    # vgchange --deltag @tag VG
  • To remove a tag from an existing LV, use:

    # lvchange --deltag @tag LV

11.5. Defining LVM host tags

This procedure describes how to define LVM host tags in a cluster configuration. You can define host tags in the configuration files.

Procedure

  • Set hosttags = 1 in the tags section to automatically define host tag using the machine’s host name.

    This allows you to use a common configuration file which can be replicated on all your machines so they hold identical copies of the file, but the behavior can differ between machines according to the host name.

For each host tag, an extra configuration file is read if it exists: lvm_hosttag.conf. If that file defines new tags, then further configuration files will be appended to the list of files to read in.

For example, the following entry in the configuration file always defines tag1, and defines tag2 if the host name is host1:

tags { tag1 { }  tag2 { host_list = ["host1"] } }

11.6. Controlling logical volume activation with tags

This procedure describes how to specify in the configuration file that only certain logical volumes should be activated on that host.

Procedure

For example, the following entry acts as a filter for activation requests (such as vgchange -ay) and only activates vg1/lvol0 and any logical volumes or volume groups with the database tag in the metadata on that host:

activation { volume_list = ["vg1/lvol0", "@database" ] }

The special match @* that causes a match only if any metadata tag matches any host tag on that machine.

As another example, consider a situation where every machine in the cluster has the following entry in the configuration file:

tags { hosttags = 1 }

If you want to activate vg1/lvol2 only on host db2, do the following:

  1. Run lvchange --addtag @db2 vg1/lvol2 from any host in the cluster.
  2. Run lvchange -ay vg1/lvol2.

This solution involves storing host names inside the volume group metadata.

Chapter 12. Troubleshooting LVM

You can use Logical Volume Manager (LVM) tools to troubleshoot a variety of issues in LVM volumes and groups.

12.1. Gathering diagnostic data on LVM

If an LVM command is not working as expected, you can gather diagnostics in the following ways.

Procedure

  • Use the following methods to gather different kinds of diagnostic data:

    • Add the -v argument to any LVM command to increase the verbosity level of the command output. Verbosity can be further increased by adding additional v’s. A maximum of four such v’s is allowed, for example, -vvvv.
    • In the log section of the /etc/lvm/lvm.conf configuration file, increase the value of the level option. This causes LVM to provide more details in the system log.
    • If the problem is related to the logical volume activation, enable LVM to log messages during the activation:

      1. Set the activation = 1 option in the log section of the /etc/lvm/lvm.conf configuration file.
      2. Execute the LVM command with the -vvvv option.
      3. Examine the command output.
      4. Reset the activation option to 0.

        If you do not reset the option to 0, the system might become unresponsive during low memory situations.

    • Display an information dump for diagnostic purposes:

      # lvmdump
    • Display additional system information:

      # lvs -v
      # pvs --all
      # dmsetup info --columns
    • Examine the last backup of the LVM metadata in the /etc/lvm/backup/ directory and archived versions in the /etc/lvm/archive/ directory.
    • Check the current configuration information:

      # lvmconfig
    • Check the /run/lvm/hints cache file for a record of which devices have physical volumes on them.

Additional resources

  • lvmdump(8) man page on your system

12.2. Displaying information about failed LVM devices

Troubleshooting information about a failed Logical Volume Manager (LVM) volume can help you determine the reason of the failure. You can check the following examples of the most common LVM volume failures.

Example 12.1. Failed volume groups

In this example, one of the devices that made up the volume group myvg failed. The volume group usability then depends on the type of failure. For example, the volume group is still usable if RAID volumes are also involved. You can also see information about the failed device.

# vgs --options +devices
 /dev/vdb1: open failed: No such device or address
 /dev/vdb1: open failed: No such device or address
  WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
  WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/sdb1).
  WARNING: Couldn't find all devices for LV myvg/mylv while checking used and assumed devices.

VG    #PV #LV #SN Attr   VSize  VFree  Devices
myvg   2   2   0 wz-pn- <3.64t <3.60t [unknown](0)
myvg   2   2   0 wz-pn- <3.64t <3.60t [unknown](5120),/dev/vdb1(0)

Example 12.2. Failed logical volume

In this example, one of the devices failed. This can be a reason for the logical volume in the volume group to fail. The command output shows the failed logical volumes.

# lvs --all --options +devices

  /dev/vdb1: open failed: No such device or address
  /dev/vdb1: open failed: No such device or address
  WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
  WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/sdb1).
  WARNING: Couldn't find all devices for LV myvg/mylv while checking used and assumed devices.

  LV    VG  Attr       LSize  Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert Devices
  mylv myvg -wi-a---p- 20.00g                                                     [unknown](0)                                                 [unknown](5120),/dev/sdc1(0)

Example 12.3. Failed image of a RAID logical volume

The following examples show the command output from the pvs and lvs utilities when an image of a RAID logical volume has failed. The logical volume is still usable.

# pvs

  Error reading device /dev/sdc1 at 0 length 4.

  Error reading device /dev/sdc1 at 4096 length 4.

  Couldn't find device with uuid b2J8oD-vdjw-tGCA-ema3-iXob-Jc6M-TC07Rn.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rimage_1 while checking used and assumed devices.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rmeta_1 while checking used and assumed devices.

  PV           VG         Fmt  Attr PSize    PFree
  /dev/sda2    rhel_bp-01 lvm2 a--  <464.76g    4.00m
  /dev/sdb1    myvg       lvm2 a--  <836.69g  736.68g
  /dev/sdd1    myvg       lvm2 a--  <836.69g <836.69g
  /dev/sde1    myvg       lvm2 a--  <836.69g <836.69g
  [unknown]    myvg       lvm2 a-m  <836.69g  736.68g
# lvs -a --options name,vgname,attr,size,devices myvg

  Couldn't find device with uuid b2J8oD-vdjw-tGCA-ema3-iXob-Jc6M-TC07Rn.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rimage_1 while checking used and assumed devices.

  WARNING: Couldn't find all devices for LV myvg/my_raid1_rmeta_1 while checking used and assumed devices.

  LV                  VG   Attr       LSize   Devices
  my_raid1            myvg rwi-a-r-p- 100.00g my_raid1_rimage_0(0),my_raid1_rimage_1(0)
  [my_raid1_rimage_0] myvg iwi-aor--- 100.00g /dev/sdb1(1)
  [my_raid1_rimage_1] myvg Iwi-aor-p- 100.00g [unknown](1)
  [my_raid1_rmeta_0]  myvg ewi-aor---   4.00m /dev/sdb1(0)
  [my_raid1_rmeta_1]  myvg ewi-aor-p-   4.00m [unknown](0)

12.3. Removing lost LVM physical volumes from a volume group

If a physical volume fails, you can activate the remaining physical volumes in the volume group and remove all the logical volumes that used that physical volume from the volume group.

Procedure

  1. Activate the remaining physical volumes in the volume group:

    # vgchange --activate y --partial myvg
  2. Check which logical volumes will be removed:

    # vgreduce --removemissing --test myvg
  3. Remove all the logical volumes that used the lost physical volume from the volume group:

    # vgreduce --removemissing --force myvg
  4. Optional: If you accidentally removed logical volumes that you wanted to keep, you can reverse the vgreduce operation:

    # vgcfgrestore myvg
    Warning

    If you remove a thin pool, LVM cannot reverse the operation.

12.4. Finding the metadata of a missing LVM physical volume

If the volume group’s metadata area of a physical volume is accidentally overwritten or otherwise destroyed, you get an error message indicating that the metadata area is incorrect, or that the system was unable to find a physical volume with a particular UUID.

This procedure finds the latest archived metadata of a physical volume that is missing or corrupted.

Procedure

  1. Find the archived metadata file of the volume group that contains the physical volume. The archived metadata files are located at the /etc/lvm/archive/volume-group-name_backup-number.vg path:

    # cat /etc/lvm/archive/myvg_00000-1248998876.vg

    Replace 00000-1248998876 with the backup-number. Select the last known valid metadata file, which has the highest number for the volume group.

  2. Find the UUID of the physical volume. Use one of the following methods.

    • List the logical volumes:

      # lvs --all --options +devices
      
        Couldn't find device with uuid 'FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk'.
    • Examine the archived metadata file. Find the UUID as the value labeled id = in the physical_volumes section of the volume group configuration.
    • Deactivate the volume group using the --partial option:

      # vgchange --activate n --partial myvg
      
        PARTIAL MODE. Incomplete logical volumes will be processed.
        WARNING: Couldn't find device with uuid 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s.
        WARNING: VG myvg is missing PV 42B7bu-YCMp-CEVD-CmKH-2rk6-fiO9-z1lf4s (last written to /dev/vdb1).
        0 logical volume(s) in volume group "myvg" now active

12.5. Restoring metadata on an LVM physical volume

This procedure restores metadata on a physical volume that is either corrupted or replaced with a new device. You might be able to recover the data from the physical volume by rewriting the metadata area on the physical volume.

Warning

Do not attempt this procedure on a working LVM logical volume. You will lose your data if you specify the incorrect UUID.

Prerequisites

Procedure

  1. Restore the metadata on the physical volume:

    # pvcreate --uuid physical-volume-uuid \
               --restorefile /etc/lvm/archive/volume-group-name_backup-number.vg \
               block-device
    Note

    The command overwrites only the LVM metadata areas and does not affect the existing data areas.

    Example 12.4. Restoring a physical volume on /dev/vdb1

    The following example labels the /dev/vdb1 device as a physical volume with the following properties:

    • The UUID of FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk
    • The metadata information contained in VG_00050.vg, which is the most recent good archived metadata for the volume group
    # pvcreate --uuid "FmGRh3-zhok-iVI8-7qTD-S5BI-MAEN-NYM5Sk" \
               --restorefile /etc/lvm/archive/VG_00050.vg \
               /dev/vdb1
    
      ...
      Physical volume "/dev/vdb1" successfully created
  2. Restore the metadata of the volume group:

    # vgcfgrestore myvg
    
      Restored volume group myvg
  3. Display the logical volumes on the volume group:

    # lvs --all --options +devices myvg

    The logical volumes are currently inactive. For example:

      LV     VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
      mylv myvg   -wi--- 300.00G                               /dev/vdb1 (0),/dev/vdb1(0)
      mylv myvg   -wi--- 300.00G                               /dev/vdb1 (34728),/dev/vdb1(0)
  4. If the segment type of the logical volumes is RAID, resynchronize the logical volumes:

    # lvchange --resync myvg/mylv
  5. Activate the logical volumes:

    # lvchange --activate y myvg/mylv
  6. If the on-disk LVM metadata takes at least as much space as what overrode it, this procedure can recover the physical volume. If what overrode the metadata went past the metadata area, the data on the volume may have been affected. You might be able to use the fsck command to recover that data.

Verification

  • Display the active logical volumes:

    # lvs --all --options +devices
    
      LV     VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
     mylv myvg   -wi--- 300.00G                               /dev/vdb1 (0),/dev/vdb1(0)
     mylv myvg   -wi--- 300.00G                               /dev/vdb1 (34728),/dev/vdb1(0)

12.6. Rounding errors in LVM output

LVM commands that report the space usage in volume groups round the reported number to 2 decimal places to provide human-readable output. This includes the vgdisplay and vgs utilities.

As a result of the rounding, the reported value of free space might be larger than what the physical extents on the volume group provide. If you attempt to create a logical volume the size of the reported free space, you might get the following error:

Insufficient free extents

To work around the error, you must examine the number of free physical extents on the volume group, which is the accurate value of free space. You can then use the number of extents to create the logical volume successfully.

12.7. Preventing the rounding error when creating an LVM volume

When creating an LVM logical volume, you can specify the number of logical extents of the logical volume to avoid rounding error.

Procedure

  1. Find the number of free physical extents in the volume group:

    # vgdisplay myvg

    Example 12.5. Free extents in a volume group

    For example, the following volume group has 8780 free physical extents:

    --- Volume group ---
     VG Name               myvg
     System ID
     Format                lvm2
     Metadata Areas        4
     Metadata Sequence No  6
     VG Access             read/write
    [...]
    Free  PE / Size       8780 / 34.30 GB
  2. Create the logical volume. Enter the volume size in extents rather than bytes.

    Example 12.6. Creating a logical volume by specifying the number of extents

    # lvcreate --extents 8780 --name mylv myvg

    Example 12.7. Creating a logical volume to occupy all the remaining space

    Alternatively, you can extend the logical volume to use a percentage of the remaining free space in the volume group. For example:

    # lvcreate --extents 100%FREE --name mylv myvg

Verification

  • Check the number of extents that the volume group now uses:

    # vgs --options +vg_free_count,vg_extent_count
    
      VG     #PV #LV #SN  Attr   VSize   VFree  Free  #Ext
      myvg   2   1   0   wz--n- 34.30G    0    0     8780

12.8. LVM metadata and their location on disk

LVM headers and metadata areas are available in different offsets and sizes.

The default LVM disk header:

  • Is found in label_header and pv_header structures.
  • Is in the second 512-byte sector of the disk. Note that if a non-default location was specified when creating the physical volume (PV), the header can also be in the first or third sector.

The standard LVM metadata area:

  • Begins 4096 bytes from the start of the disk.
  • Ends 1 MiB from the start of the disk.
  • Begins with a 512 byte sector containing the mda_header structure.

A metadata text area begins after the mda_header sector and goes to the end of the metadata area. LVM VG metadata text is written in a circular fashion into the metadata text area. The mda_header points to the location of the latest VG metadata within the text area.

You can print LVM headers from a disk by using the # pvck --dump headers /dev/sda command. This command prints label_header, pv_header, mda_header, and the location of metadata text if found. Bad fields are printed with the CHECK prefix.

The LVM metadata area offset will match the page size of the machine that created the PV, so the metadata area can also begin 8K, 16K or 64K from the start of the disk.

Larger or smaller metadata areas can be specified when creating the PV, in which case the metadata area may end at locations other than 1 MiB. The pv_header specifies the size of the metadata area.

When creating a PV, a second metadata area can be optionally enabled at the end of the disk. The pv_header contains the locations of the metadata areas.

12.9. Extracting VG metadata from a disk

Choose one of the following procedures to extract VG metadata from a disk, depending on your situation. For information about how to save extracted metadata, see Saving extracted metadata to a file.

Note

For repair, you can use backup files in /etc/lvm/backup/ without extracting metadata from disk.

Procedure

  • Print current metadata text as referenced from valid mda_header:

    # pvck --dump metadata <disk>

    Example 12.8. Metadata text from valid mda_header

    # pvck --dump metadata /dev/sdb
      metadata text at 172032 crc Oxc627522f # vgname test segno 59
      ---
      <raw metadata from disk>
      ---
  • Print the locations of all metadata copies found in the metadata area, based on finding a valid mda_header:

    # pvck --dump metadata_all <disk>

    Example 12.9. Locations of metadata copies in the metadata area

    # pvck --dump metadata_all /dev/sdb
      metadata at 4608 length 815 crc 29fcd7ab vg test seqno 1 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 7168 length 1450 crc 5652ea55 vg test seqno 3 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
  • Search for all copies of metadata in the metadata area without using an mda_header, for example, if headers are missing or damaged:

    # pvck --dump metadata_search <disk>

    Example 12.10. Copies of metadata in the metadata area without using an mda_header

    # pvck --dump metadata_search /dev/sdb
      Searching for metadata at offset 4096 size 1044480
      metadata at 4608 length 815 crc 29fcd7ab vg test seqno 1 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
      metadata at 7168 length 1450 crc 5652ea55 vg test seqno 3 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
  • Include the -v option in the dump command to show the description from each copy of metadata:

    # pvck --dump metadata -v <disk>

    Example 12.11. Showing description from each copy of metadata

    # pvck --dump metadata -v /dev/sdb
      metadata text at 199680 crc 0x628cf243 # vgname my_vg seqno 40
      ---
    my_vg {
    id = "dmEbPi-gsgx-VbvS-Uaia-HczM-iu32-Rb7iOf"
    seqno = 40
    format = "lvm2"
    status = ["RESIZEABLE", "READ", "WRITE"]
    flags = []
    extent_size = 8192
    max_lv = 0
    max_pv = 0
    metadata_copies = 0
    
    physical_volumes {
    
    pv0 {
    id = "8gn0is-Hj8p-njgs-NM19-wuL9-mcB3-kUDiOQ"
    device = "/dev/sda"
    
    device_id_type = "sys_wwid"
    device_id = "naa.6001405e635dbaab125476d88030a196"
    status = ["ALLOCATABLE"]
    flags = []
    dev_size = 125829120
    pe_start = 8192
    pe_count = 15359
    }
    
    pv1 {
    id = "E9qChJ-5ElL-HVEp-rc7d-U5Fg-fHxL-2QLyID"
    device = "/dev/sdb"
    
    device_id_type = "sys_wwid"
    device_id = "naa.6001405f3f9396fddcd4012a50029a90"
    status = ["ALLOCATABLE"]
    flags = []
    dev_size = 125829120
    pe_start = 8192
    pe_count = 15359
    }

This file can be used for repair. The first metadata area is used by default for dump metadata. If the disk has a second metadata area at the end of the disk, you can use the --settings "mda_num=2" option to use the second metadata area for dump metadata instead.

12.10. Saving extracted metadata to a file

If you need to use dumped metadata for repair, it is required to save extracted metadata to a file with the -f option and the --setings option.

Procedure

  • If -f <filename> is added to --dump metadata, the raw metadata is written to the named file. You can use this file for repair.
  • If -f <filename> is added to --dump metadata_all or --dump metadata_search, then raw metadata from all locations is written to the named file.
  • To save one instance of metadata text from --dump metadata_all|metadata_search add --settings "metadata_offset=<offset>" where <offset> is from the listing output "metadata at <offset>".

    Example 12.12. Output of the command

    # pvck --dump metadata_search --settings metadata_offset=5632 -f meta.txt /dev/sdb
      Searching for metadata at offset 4096 size 1044480
      metadata at 5632 length 1144 crc 50ea61c3 vg test seqno 2 id FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv
    # head -2 meta.txt
    test {
    id = "FaCsSz-1ZZn-mTO4-Xl4i-zb6G-BYat-u53Fxv"

12.11. Repairing a disk with damaged LVM headers and metadata using the pvcreate and the vgcfgrestore commands

You can restore metadata and headers on a physical volume that is either corrupted or replaced with a new device. You might be able to recover the data from the physical volume by rewriting the metadata area on the physical volume.

Warning

These instructions should be used with extreme caution, and only if you are familiar with the implications of each command, the current layout of the volumes, the layout that you need to achieve, and the contents of the backup metadata file. These commands have the potential to corrupt data, and as such, it is recommended that you contact Red Hat Global Support Services for assistance in troubleshooting.

Prerequisites

Procedure

  1. Collect the following information needed for the pvcreate and vgcfgrestore commands. You can collect the information about your disk and UUID by running the # pvs -o+uuid command.

    • metadata-file is the path to the most recent metadata backup file for the VG, for example, /etc/lvm/backup/<vg-name>
    • vg-name is the name of the VG that has the damaged or missing PV.
    • UUID of the PV that was damaged on this device is the value taken from the output of the # pvs -i+uuid command.
    • disk is the name of the disk where the PV is supposed to be, for example, /dev/sdb. Be certain this is the correct disk, or seek help, otherwise following these steps may lead to data loss.
  2. Recreate LVM headers on the disk:

    # pvcreate --restorefile <metadata-file> --uuid <UUID> <disk>

    Optionally, verify that the headers are valid:

    # pvck --dump headers <disk>
  3. Restore the VG metadata on the disk:

    # vgcfgrestore --file <metadata-file> <vg-name>

    Optionally, verify the metadata is restored:

    # pvck --dump metadata <disk>

If there is no metadata backup file for the VG, you can get one by using the procedure in Saving extracted metadata to a file.

Verification

  • To verify that the new physical volume is intact and the volume group is functioning correctly, check the output of the following command:
# vgs

12.12. Repairing a disk with damaged LVM headers and metadata using the pvck command

This is an alternative to the Repairing a disk with damaged LVM headers and metadata using the pvcreate and the vgcfgrestore commands. There may be cases where the pvcreate and the vgcfgrestore commands do not work. This method is more targeted at the damaged disk.

This method uses a metadata input file that was extracted by pvck --dump, or a backup file from /etc/lvm/backup. When possible, use metadata saved by pvck --dump from another PV in the same VG, or from a second metadata area on the PV. For more information, see Saving extracted metadata to a file.

Procedure

  • Repair the headers and metadata on the disk:

    # pvck --repair -f <metadata-file> <disk>

    where

    • <metadata-file> is a file containing the most recent metadata for the VG. This can be /etc/lvm/backup/vg-name, or it can be a file containing raw metadata text from the pvck --dump metadata_search command output.
    • <disk> is the name of the disk where the PV is supposed to be, for example, /dev/sdb. To prevent data loss, verify that is the correct disk. If you are not certain the disk is correct, contact Red Hat Support.
Note

If the metadata file is a backup file, the pvck --repair should be run on each PV that holds metadata in VG. If the metadata file is raw metadata that has been extracted from another PV, the pvck --repair needs to be run only on the damaged PV.

Verification

  • To check that the new physical volume is intact and the volume group is functioning correctly, check outputs of the following commands:

    # vgs <vgname>
    # pvs <pvname>
    # lvs <lvname>

12.13. Troubleshooting LVM RAID

You can troubleshoot various issues in LVM RAID devices to correct data errors, recover devices, or replace failed devices.

12.13.1. Checking data coherency in a RAID logical volume

LVM provides scrubbing support for RAID logical volumes. RAID scrubbing is the process of reading all the data and parity blocks in an array and checking to see whether they are coherent. The lvchange --syncaction repair command initiates a background synchronization action on the array. The following attributes provide details about data coherency:

  • The raid_sync_action field displays the current synchronization action that the RAID logical volume is performing. It can be one of the following values:

    idle
    Completed all sync actions (doing nothing).
    resync
    Initializing or resynchronizing an array after an unclean machine shutdown.
    recover
    Replacing a device in the array.
    check
    Looking for array inconsistencies.
    repair
    Looking for and repairing inconsistencies.
  • The raid_mismatch_count field displays the number of discrepancies found during a check action.
  • The Cpy%Sync field displays the progress of the sync actions.
  • The lv_attr field provides additional indicators. Bit 9 of this field displays the health of the logical volume, and it supports the following indicators:

    m or mismatches
    Indicates that there are discrepancies in a RAID logical volume. You can see this character after the scrubbing operation detects the portions of the RAID, which are not coherent.
    r or refresh
    Indicates a failed device in a RAID array, even though LVM can read the device label and considers the device to be operational. Refresh the logical volume to notify the kernel that the device is now available, or replace the device if you suspect that it failed.

Procedure

  1. Optional: Limit the I/O bandwidth that the scrubbing process uses. When you perform a RAID scrubbing operation, the background I/O required by the sync actions can crowd out other I/O to LVM devices, such as updates to volume group metadata. This might cause the other LVM operations to slow down.

    You can control the rate of the scrubbing operation by implementing recovery throttling. You can set the recovery rate using --maxrecoveryrate Rate[bBsSkKmMgG] or --minrecoveryrate Rate[bBsSkKmMgG] with the lvchange --syncaction commands. For more information, see Minimum and maximum I/O rate options.

    Specify the Rate value as an amount per second for each device in the array. If you provide no suffix, the options assume kiB per second per device.

  2. Display the number of discrepancies in the array, without repairing them:

    # lvchange --syncaction check my_vg/my_lv

    This command initiates a background synchronization action on the array.

  3. Optional: View the var/log/syslog file for the kernel messages.
  4. Correct the discrepancies in the array:

    # lvchange --syncaction repair my_vg/my_lv

    This command repairs or replaces failed devices in a RAID logical volume. You can view the var/log/syslog file for the kernel messages after executing this command.

Verification

  1. Display information about the scrubbing operation:

    # lvs -o +raid_sync_action,raid_mismatch_count my_vg/my_lv
    LV    VG    Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert SyncAction Mismatches
    my_lv my_vg rwi-a-r--- 500.00m                                    100.00           idle        0

Additional resources

12.13.2. Replacing a failed RAID device in a logical volume

RAID is not similar to traditional LVM mirroring. In case of LVM mirroring, remove the failed devices. Otherwise, the mirrored logical volume would hang while RAID arrays continue running with failed devices. For RAID levels other than RAID1, removing a device would mean converting to a lower RAID level, for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0.

Instead of removing a failed device and allocating a replacement, with LVM, you can replace a failed device that serves as a physical volume in a RAID logical volume by using the --repair argument of the lvconvert command.

Prerequisites

  • The volume group includes a physical volume that provides enough free capacity to replace the failed device.

    If no physical volume with enough free extents is available on the volume group, add a new, sufficiently large physical volume by using the vgextend utility.

Procedure

  1. View the RAID logical volume:

    # lvs --all --options name,copy_percent,devices my_vg
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdc1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdc1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  2. View the RAID logical volume after the /dev/sdc device fails:

    # lvs --all --options name,copy_percent,devices my_vg
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
      LV               Cpy%Sync Devices
      my_lv            100.00   my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          [unknown](1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           [unknown](0)
      [my_lv_rmeta_2]           /dev/sdd1(0)
  3. Replace the failed device:

    # lvconvert --repair my_vg/my_lv
      /dev/sdc: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
      WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
    Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
    Faulty devices in my_vg/my_lv successfully replaced.
  4. Optional: Manually specify the physical volume that replaces the failed device:

    # lvconvert --repair my_vg/my_lv replacement_pv
  5. Examine the logical volume with the replacement:

    # lvs --all --options name,copy_percent,devices my_vg
    
      /dev/sdc: open failed: No such device or address
      /dev/sdc1: open failed: No such device or address
      Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
      LV               Cpy%Sync Devices
      my_lv            43.79    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

    Until you remove the failed device from the volume group, LVM utilities still indicate that LVM cannot find the failed device.

  6. Remove the failed device from the volume group:

    # vgreduce --removemissing my_vg

Verification

  1. View the available physical volumes after removing the failed device:

    # pvscan
    PV /dev/sde1 VG rhel_virt-506 lvm2 [<7.00 GiB / 0 free]
    PV /dev/sdb1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
    PV /dev/sdd1 VG my_vg lvm2 [<60.00 GiB / 59.50 GiB free]
  2. Examine the logical volume after the replacing the failed device:

    # lvs --all --options name,copy_percent,devices my_vg
    my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
      [my_lv_rimage_0]          /dev/sde1(1)
      [my_lv_rimage_1]          /dev/sdb1(1)
      [my_lv_rimage_2]          /dev/sdd1(1)
      [my_lv_rmeta_0]           /dev/sde1(0)
      [my_lv_rmeta_1]           /dev/sdb1(0)
      [my_lv_rmeta_2]           /dev/sdd1(0)

Additional resources

  • lvconvert(8) and vgreduce(8) man pages on your system

12.14. Troubleshooting duplicate physical volume warnings for multipathed LVM devices

When using LVM with multipathed storage, LVM commands that list a volume group or logical volume might display messages such as the following:

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/dm-5 not /dev/sdd
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/emcpowerb not /dev/sde
Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/sddlmab not /dev/sdf

You can troubleshoot these warnings to understand why LVM displays them, or to hide the warnings.

12.14.1. Root cause of duplicate PV warnings

When a multipath software such as Device Mapper Multipath (DM Multipath), EMC PowerPath, or Hitachi Dynamic Link Manager (HDLM) manages storage devices on the system, each path to a particular logical unit (LUN) is registered as a different SCSI device.

The multipath software then creates a new device that maps to those individual paths. Because each LUN has multiple device nodes in the /dev directory that point to the same underlying data, all the device nodes contain the same LVM metadata.

Table 12.1. Example device mappings in different multipath software
Multipath softwareSCSI paths to a LUNMultipath device mapping to paths

DM Multipath

/dev/sdb and /dev/sdc

/dev/mapper/mpath1 or /dev/mapper/mpatha

EMC PowerPath

/dev/emcpowera

HDLM

/dev/sddlmab

As a result of the multiple device nodes, LVM tools find the same metadata multiple times and report them as duplicates.

12.14.2. Cases of duplicate PV warnings

LVM displays the duplicate PV warnings in either of the following cases:

Single paths to the same device

The two devices displayed in the output are both single paths to the same device.

The following example shows a duplicate PV warning in which the duplicate devices are both single paths to the same device.

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/sdd not /dev/sdf

If you list the current DM Multipath topology using the multipath -ll command, you can find both /dev/sdd and /dev/sdf under the same multipath map.

These duplicate messages are only warnings and do not mean that the LVM operation has failed. Rather, they are alerting you that LVM uses only one of the devices as a physical volume and ignores the others.

If the messages indicate that LVM chooses the incorrect device or if the warnings are disruptive to users, you can apply a filter. The filter configures LVM to search only the necessary devices for physical volumes, and to leave out any underlying paths to multipath devices. As a result, the warnings no longer appear.

Multipath maps

The two devices displayed in the output are both multipath maps.

The following examples show a duplicate PV warning for two devices that are both multipath maps. The duplicate physical volumes are located on two different devices rather than on two different paths to the same device.

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/mapper/mpatha not /dev/mapper/mpathc

Found duplicate PV GDjTZf7Y03GJHjteqOwrye2dcSCjdaUi: using /dev/emcpowera not /dev/emcpowerh

This situation is more serious than duplicate warnings for devices that are both single paths to the same device. These warnings often mean that the machine is accessing devices that it should not access: for example, LUN clones or mirrors.

Unless you clearly know which devices you should remove from the machine, this situation might be unrecoverable. Red Hat recommends that you contact Red Hat Technical Support to address this issue.

12.14.3. Example LVM device filters that prevent duplicate PV warnings

The following examples show LVM device filters that avoid the duplicate physical volume warnings that are caused by multiple storage paths to a single logical unit (LUN).

You can configure the filter for logical volume manager (LVM) to check metadata for all devices. Metadata includes local hard disk drive with the root volume group on it and any multipath devices. By rejecting the underlying paths to a multipath device (such as /dev/sdb, /dev/sdd), you can avoid these duplicate PV warnings, because LVM finds each unique metadata area once on the multipath device itself.

  • To accept the second partition on the first hard disk drive and any device mapper (DM) Multipath devices and reject everything else, enter:

    filter = [ "a|/dev/sda2$|", "a|/dev/mapper/mpath.*|", "r|.*|" ]
  • To accept all HP SmartArray controllers and any EMC PowerPath devices, enter:

    filter = [ "a|/dev/cciss/.*|", "a|/dev/emcpower.*|", "r|.*|" ]
  • To accept any partitions on the first IDE drive and any multipath devices, enter:

    filter = [ "a|/dev/hda.*|", "a|/dev/mapper/mpath.*|", "r|.*|" ]

12.14.4. Additional resources

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