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5.4.5. Creating Snapshot Volumes

Note

As of the Red Hat Enterprise Linux 6.4 release, LVM supports thinly-provisioned snapshots. For information on creating thinly provisioned snapshot volumes, see Section 5.4.6, “Creating Thinly-Provisioned Snapshot Volumes”.
Use the -s argument of the lvcreate command to create a snapshot volume. A snapshot volume is writable.

Note

LVM snapshots are not supported across the nodes in a cluster. You cannot create a snapshot volume in a clustered volume group. As of the Red Hat Enterprise Linux 6.1 release, however, 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”.

Note

As of the Red Hat Enterprise Linux 6.1 release, LVM snapshots are supported for mirrored logical volumes.
As of the Red Hat Enterprise Linux 6.3 release, snapshots are supported for RAID logical volumes. For information on RAID logical volumes, see Section 5.4.16, “RAID Logical Volumes”.
As of the Red Hat Enterprise Linux 6.5 release, LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. If you specify a snapshot volume that is larger than this, the system will create a snapshot volume that is only as large as will be needed for the size of the origin.
By default, a snapshot volume is skipped during normal activation commands. For information on controlling the activation of a snapshot volume, see Section 5.4.17, “Controlling Logical Volume Activation”.
The following command creates a snapshot logical volume that is 100 MB in size named /dev/vg00/snap. This creates a snapshot of the origin logical volume named /dev/vg00/lvol1. If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in order to access the contents of the file system to run a backup while the original file system continues to get updated. 
# lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
After you create a snapshot logical volume, specifying the origin volume on the lvdisplay command yields output that includes a list of all snapshot logical volumes and their status (active or inactive).
The following example shows the status of the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created. 
# lvdisplay /dev/new_vg/lvol0
  --- Logical volume ---
  LV Name                /dev/new_vg/lvol0
  VG Name                new_vg
  LV UUID                LBy1Tz-sr23-OjsI-LT03-nHLC-y8XW-EhCl78
  LV Write Access        read/write
  LV snapshot status     source of
                         /dev/new_vg/newvgsnap1 [active]
  LV Status              available
  # open                 0
  LV Size                52.00 MB
  Current LE             13
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:2
The lvs command, by default, displays the origin volume and the current percentage of the snapshot volume being used for each snapshot volume. The following example shows the default output for the lvs command for a system that includes the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created. 
# lvs
  LV         VG     Attr   LSize  Origin Snap%  Move Log Copy%
  lvol0      new_vg owi-a- 52.00M
  newvgsnap1 new_vg swi-a-  8.00M lvol0    0.20

Warning

Because the snapshot increases in size as the origin volume changes, it is important to monitor the percentage of the snapshot volume regularly with the lvs command to be sure it does not fill. A snapshot that is 100% full is lost completely, as a write to unchanged parts of the origin would be unable to succeed without corrupting the snapshot.
As of the Red Hat Enterprise Linux 6.2 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.
As of the Red Hat Enterprise Linux 6.5 release, LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. Similarly, automatic extension of a snapshot will not increase the size of a snapshot volume beyond the maximum calculated size that is necessary for the snapshot. Once a snapshot has grown large enough to cover the origin, it is no longer monitored for automatic extension.
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, see Appendix B, The LVM Configuration Files.