You can also take instant snapshots of RAID-5 volumes that have been converted to a special layered volume layout by the addition of a DCO and DCO volume.
A plex in a full-sized instant snapshot requires as much space as the original volume. If you instead make a space-optimized instant snapshot of a volume, this only requires enough storage to record the original contents of the parent volume as they are changed during the life of the snapshot.
The recommended approach to performing volume backup from the command line, or from a script, is to use the vxsnap command. The vxsnap prepare and make tasks allow you to back up volumes online with minimal disruption to users.
vxsnap prepare creates a DCO and DCO volume and associates this with the original volume. It also enables Persistent FastResync.
vxsnap make creates an instant snapshot that is immediately available for making a backup. After the snapshot has been taken, read requests for data in the instant snapshot volume are satisfied by reading either from a non-updated region of the original volume, or from the copy of the original contents of an updated region that have been recorded by the snapshot.
Synchronization of a full-sized instant snapshot from the original volume is enabled by default. If you specify the syncing=no attribute to vxsnap make, this disables synchronization, and the contents of the instant snapshot are unlikely ever to become fully synchronized with the contents of the original volume at the point in time that the snapshot was taken. In such a case, the snapshot cannot be used for off-host processing, nor can it become an independent volume.
You can immediately retake a full-sized or space-optimized instant snapshot at any time by using the vxsnap refresh command. If a fully synchronized instant snapshot is required, the new resynchronization must first complete.
To create instant snapshots of volume sets, use volume set names in place of volume names in the vxsnap command.
When using the vxsnap prepare or vxassist make commands to make a volume ready for instant snapshot operations, if the specified region size exceeds half the value of the tunable voliomem_maxpool_sz , the operation succeeds but gives a warning such as the following (for a system where voliomem_maxpool_sz is set to 12MB):
VxVM vxassist WARNING V-5-1-0 Specified regionsize is larger than the limit on the system (voliomem_maxpool_sz/2=6144k).
If this message is displayed, vxsnap make, refresh and restore operations on such volumes fail as they might potentially hang the system. Such volumes can be used only for break-off snapshot operations using the reattach and make operations.
To make the volumes usable for instant snapshot operations, use vxsnap unprepare on the volume, and then use vxsnap prepare to re-prepare the volume with a region size that is less than half the size of voliomem_maxpool_sz (in this example, 1MB):
# vxsnap -g mydg -f unprepare vol1 # vxsnap -g mydg prepare vol1 regionsize=1M