Chapter 2. The LVM Logical Volume Manager

You can define how a mirrored logical volume behaves in the event of a device failure with the mirror_image_fault_policy and mirror_log_fault_policy parameters in the activation section of the lvm.conf file. When this parameter is set to remove, the system attempts to remove the faulty device and run without it. When this parameter is set to allocate, the system attempts to remove the faulty device and tries to allocate space on a new device to be a replacement for the failed device; this policy acts like the remove policy if no suitable device and space can be allocated for the replacement. For information on the LVM mirror failure policies, see Section 5.4.3.1, “Mirrored Logical Volume Failure Policy”.

For the Red Hat Enterprise Linux 6 release, the Linux I/O stack has been enhanced to process vendor-provided I/O limit information. This allows storage management tools, including LVM, to optimize data placement and access. This support can be disabled by changing the default values of data_alignment_detection and data_alignment_offset_detection in the lvm.conf file, although disabling this support is not recommended.

For information on data alignment in LVM as well as information on changing the default values of data_alignment_detection and data_alignment_offset_detection, see the inline documentation for the /etc/lvm/lvm.conf file, which is also documented in Appendix B, The LVM Configuration Files. For general information on support for the I/O Stack and I/O limits in Red Hat Enterprise Linux 6, see the Storage Administration Guide.

In Red Hat Enterprise Linux 6, the Device Mapper provides direct support for udev integration. This synchronizes the Device Mapper with all udev processing related to Device Mapper devices, including LVM devices. For information on Device Mapper support for the udev device manager, see Section A.3, “Device Mapper Support for the udev Device Manager”.

For the Red Hat Enterprise Linux 6 release, you can use the lvconvert --repair command to repair a mirror after disk failure. This brings the mirror back into a consistent state. For information on the lvconvert --repair command, see Section 5.4.3.3, “Repairing a Mirrored Logical Device”.

As of the Red Hat Enterprise Linux 6 release, you can use the --merge option of the lvconvert command to merge a snapshot into its origin volume. For information on merging snapshots, see Section 5.4.8, “Merging Snapshot Volumes”.

As of the Red Hat Enterprise Linux 6 release, you can use the --splitmirrors argument of the lvconvert command to split off a redundant image of a mirrored logical volume to form a new logical volume. For information on using this option, see Section 5.4.3.2, “Splitting Off a Redundant Image of a Mirrored Logical Volume”.

You can now create a mirror log for a mirrored logical device that is itself mirrored by using the --mirrorlog mirrored argument of the lvcreate command when creating a mirrored logical device. For information on using this option, see Section 5.4.3, “Creating Mirrored Volumes”.

The Red Hat Enterprise Linux 6.1 release supports the creation of snapshot logical volumes of mirrored logical volumes. You create a snapshot of a mirrored volume just as you would create a snapshot of a linear or striped logical volume. For information on creating snapshot volumes, see Section 5.4.5, “Creating Snapshot Volumes”.

When extending an LVM volume, you can now use the --alloc cling option of the lvextend command to specify the cling allocation policy. This policy will choose space on the same physical volumes as the last segment of the existing logical volume. If there is insufficient space on the physical volumes and a list of tags is defined in the lvm.conf file, LVM will check whether any of the tags are attached to the physical volumes and seek to match those physical volume tags between existing extents and new extents.

For information on extending LVM mirrored volumes with the --alloc cling option of the lvextend command, see Section 5.4.14.3, “Extending a Logical Volume with the cling Allocation Policy”.

You can now specify multiple --addtag and --deltag arguments within a single pvchange, vgchange, or lvchange command. For information on adding and removing object tags, see Section D.1, “Adding and Removing Object Tags”.

The list of allowed characters in LVM object tags has been extended, and tags can contain the "/", "=", "!", ":", "#", and "&" characters. For information on LVM object tags, see Appendix D, LVM Object Tags.

You can now combine RAID0 (striping) and RAID1 (mirroring) in a single logical volume. Creating a logical volume while simultaneously specifying the number of mirrors (--mirrors X) and the number of stripes (--stripes Y) results in a mirror device whose constituent devices are striped. For information on creating mirrored logical volumes, see Section 5.4.3, “Creating Mirrored Volumes”.

As of the Red Hat Enterprise Linux 6.1 release, if you need to create a consistent backup of data on a clustered logical volume you can activate the volume exclusively and then create the snapshot. For information on activating logical volumes exclusively on one node, see Section 5.7, “Activating Logical Volumes on Individual Nodes in a Cluster”.

The Red Hat Enterprise Linux 6.2 release supports the issue_discards parameter in the lvm.conf configuration file. When this parameter is set, LVM will issue discards to a logical volume's underlying physical volumes when the logical volume is no longer using the space on the physical volumes. For information on this parameter, see the inline documentation for the /etc/lvm/lvm.conf file, which is also documented in Appendix B, The LVM Configuration Files.

As of the Red Hat Enterprise Linux 6.3 release, LVM supports RAID4/5/6 and a new implementation of mirroring. For information on RAID logical volumes, see Section 5.4.16, “RAID Logical Volumes”.

When you are creating a new mirror that does not need to be revived, you can specify the --nosync argument to indicate that an initial synchronization from the first device is not required. For information on creating mirrored volumes, see Section 5.4.3, “Creating Mirrored Volumes”.

This manual now documents the snapshot autoextend feature. For information on creating snapshot volumes, see Section 5.4.5, “Creating Snapshot Volumes”.

Logical volumes can now be thinly provisioned. This allows you to create logical volumes that are larger than the available extents. Using thin provisioning, you can manage a storage pool of free space, known as a thin pool, to be allocated to an arbitrary number of devices when needed by applications. You can then create devices that can be bound to the thin pool for later allocation when an application actually writes to the logical volume. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space.

For general information on thinly-provisioned logical volumes, see Section 3.3.5, “Thinly-Provisioned Logical Volumes (Thin Volumes)”. For information on creating thin volumes, see Section 5.4.4, “Creating Thinly-Provisioned Logical Volumes”.

The Red Hat Enterprise Linux release 6.4 version of LVM provides support for thinly-provisioned snapshot volumes. Thin snapshot volumes allow many virtual devices to be stored on the same data volume. This simplifies administration and allows for the sharing of data between snapshot volumes.

For general information on thinly-provisioned snapshot volumes, see Section 3.3.7, “Thinly-Provisioned Snapshot Volumes”. For information on creating thin snapshot volumes, see Section 5.4.6, “Creating Thinly-Provisioned Snapshot Volumes”.

This document includes a new section detailing LVM allocation policy, Section 5.3.2, “LVM Allocation”.

LVM now provides support for raid10 logical volumes. For information on RAID logical volumes, see Section 5.4.16, “RAID Logical Volumes”.

The LVM metadata daemon, lvmetad, is supported in Red Hat Enterprise Linux release 6.4. Enabling this daemon reduces the amount of scanning on systems with many block devices. The lvmetad daemon is not currently supported across the nodes of a cluster, and requires that the locking type be local file-based locking.

For information on the metadata daemon, see Section 4.6, “The Metadata Daemon (lvmetad)”.

You can control I/O operations on a RAID1 logical volume with the --writemostly and --writebehind parameters of the lvchange command. For information on these parameters, see Section 5.4.16.11, “Controlling I/O Operations on a RAID1 Logical Volume”.

The lvchange command now supports a --refresh parameter that allows you to restore a transiently failed device without having to reactivate the device. This feature is described in Section 5.4.16.8.1, “The allocate RAID Fault Policy”.

LVM provides scrubbing support for RAID logical volumes. For information on this feature, see Section 5.4.16.10, “Scrubbing a RAID Logical Volume”.

The fields that the lvs command supports have been updated. For information on the lvs command, see Table 5.4, “lvs Display Fields”.

The lvchange command supports the new --maxrecoveryrate and --minrecoveryrate parameters, which allow you to control the rate at which sync operations are performed. For information on these parameters, see Section 5.4.16.10, “Scrubbing a RAID Logical Volume”.

You can control the rate at which a RAID logical volume is initialized by implementing recovery throttling. You control the rate at which sync operations are performed by setting the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvcreate command, as described in Section 5.4.16.1, “Creating a RAID Logical Volume”.

You can now create a thinly-provisioned snapshot of a non-thinly-provisioned logical volume. For information on creating these volumes, known as external volumes, see Section 3.3.7, “Thinly-Provisioned Snapshot Volumes”.

The documentation for thinly-provisioned volumes and thinly-provisioned snapshots has been clarified. Additional information about LVM thin provisioning is now provided in the lvmthin(7) man page. For general information on thinly-provisioned logical volumes, see Section 3.3.5, “Thinly-Provisioned Logical Volumes (Thin Volumes)”. For information on thinly-provisioned snapshot volumes, see Section 3.3.7, “Thinly-Provisioned Snapshot Volumes”.

This manual now documents the lvm dumpconfig command, in Section B.2, “The lvmconfig Command”. Note that as of the Red Hat Enterprise Linux 6.8 release, this command was renamed lvmconf, although the old format continues to work.

This manual now documents LVM profiles, in Section B.3, “LVM Profiles”.

This manual now documents the lvm command in Section 4.7, “Displaying LVM Information with the lvm Command”.

In the Red Hat Enterprise Linux 6.6 release, you can control activation of thin pool snapshots with the -k and -K options of the lvcreate and lvchange command, as documented in Section 5.4.17, “Controlling Logical Volume Activation”.

This manual documents the --force argument of the vgimport command. This allows you to import volume groups that are missing physical volumes and subsequently run the vgreduce --removemissing command. For information on the vgimport command, see Section 5.3.15, “Moving a Volume Group to Another System”.

As of Red Hat Enterprise Linux release 6.7, many LVM processing commands accept the -S or --select option to define selection criteria for those commands. LVM selection criteria are documented in the new appendix Appendix C, LVM Selection Criteria.

This document provides basic procedures for creating cache logical volumes in Section 5.4.7, “Creating LVM Cache Logical Volumes”.

The troubleshooting chapter of this document includes a new section, Section 7.8, “Duplicate PV Warnings for Multipathed Devices”.

When defining selection criteria for LVM commands, you can now specify time values as selection criteria for fields with a field type of time. For information on specifying time values as selection criteria, see Section C.3.1, “Specifying Time Values”.