Chapter 2. The LVM Logical Volume Manager

In Red Hat Enterprise Linux 5.6, the lvm.conf configuration file includes a default_data_alignment parameter that allows you to set the default alignment of the start of a data area. For information on data alignment in LVM as well as information on changing the default value of default_data_alignment, see the inline documentation for the /etc/lvm/lvm.conf file, which is also documented in Appendix B, The LVM Configuration Files.

In the Red Hat Enterprise Linux 5.6 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 5.4.1.3.3, “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 5.4.1.3.1, “Mirroring the Mirror Log”.

In the Red Hat Enterprise Linux 5.6 release, you can control metadata storage on a physical volume by using the --metadataignore option of the pvcreate command. For information on setting this value, see Appendix D, LVM Volume Group Metadata and the pvcreate man page.

In the Red Hat Enterprise Linux 5.6 release, you can determine the number of metadata copies at the volume group level with the --vgmetadatacopies option of the vgcreate command. For information on setting this value, see Appendix D, LVM Volume Group Metadata and the vgcreate man page.

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 5.4.1.3, “Creating Mirrored Volumes”.

As of the Red Hat Enterprise Linux 5.7 release, there are two new features related to snapshots. First, in addition to the snapshot itself being invalidated when full, any mounted file systems on that snapshot device are forcibly unmounted, avoiding the inevitable file system errors upon access to the mount point. Second, you can specify the snapshot_autoextend_threshold option in the lvm.conf file. This option allows automatic extension of a snapshot whenever the remaining snapshot space drops below the threshold you set. This feature requires that there be unallocated space in the volume group.

Information on setting snapshot_autoextend_threshold and snapshot_autoextend_percent is provided in the lvm.conf file itself. For information about the lvm.conf file, refer to Appendix B, The LVM Configuration Files.

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 5.4.11, “Extending a Logical Volume with the cling Allocation Policy”.

As of the Red Hat Enterprise Linux 5.7 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 5.8, “Activating Logical Volumes on Individual Nodes in a Cluster”.

As of the Red Hat Enterprise Linux 5.8 release, it is possible to grow mirrored logical volumes with the lvextend command without performing a synchronization of the new mirror regions. For information on extending a mirrored logical volume, see Section 5.4.10, “Extending a Mirrored Volume”.

Small clarifactions have been made throughout this document.

Small clarifactions have been made throughout this document.

This manual documents the lvm tags command, which lists the currently active host tags. For information on LVM object tags, see Appendix C, LVM Object Tags.

As of the Red Hat Enterprise Linux 5.11 release, you no longer need to specify a major number when using the --persistent option of the lvcreate or lvchange command. This had no effect on the major number, which the kernel assigned dynamically. For information on persistent device numbers, see Section 5.4.2, “Persistent Device Numbers”.