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Creating instant snapshots

VxVM allows you to make instant snapshots of volumes by using the vxsnap command.

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.

See "Using a DCO and DCO volume with a RAID-5 volume" on page 286.

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.

The vxsnap prepare step creates a DCO and DCO volume and associates this with the volume. It also enables Persistent FastResync on the volume.

The vxsnap make step creates an instant snapshot that is immediately available for making a backup. After the snapshot has been taken, read requests for data in the original 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.

  Note   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. If you wish to move an instant snapshot volume to another disk group for export to another machine for off-host processing, or to turn it into an independent volume, you must ensure that the snapshot volume has been completely synchronized.

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, you must wait for the new resynchronization to complete.

You can create instant snapshots of volume sets by replacing volume names with volume set names in the vxsnap command.

See "Creating instant snapshots of volume sets" on page 340.

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

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

See "Preparing to create instant and break-off snapshots" on page 327.

See "Creating and managing space-optimized instant snapshots" on page 330.

See "Creating and managing full-sized instant snapshots" on page 332.

See "Creating and managing third-mirror break-off snapshots" on page 334.

See "Creating and managing linked break-off snapshot volumes" on page 337.