Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

311Administering volume snapshots
Creating instant snapshots
Note: 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 (see “voliomem_maxpool_sz” on page 469), 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
To create and manage a snapshot of a volume with the vxsnap command, follow the
procedure in “Preparing to create instant and break-off snapshots” on page 311, and then
use one of the procedures described in the following sections:
■ “Creating and managing space-optimized instant snapshots” on page 315
■ “Creating and managing full-sized instant snapshots” on page 317
■ “Creating and managing third-mirror break-off snapshots” on page 319
■ “Creating and managing linked break-off snapshot volumes” on page 321
Preparing to create instant and break-off snapshots
To prepare a volume for the creation of instant and break-off snapshots
1 Use the following commands to see if the volume is associated with a version 20 data
change object (DCO) and DCO volume that allow instant snapshots and Persistent
FastResync to be used with the volume, and to check that FastResync is enabled on
the volume:
# vxprint -g volumedg -F%instant volume
# vxprint -g volumedg -F%fastresync volume
If both commands return a value of on, the volume can be used for instant snapshot
operations, and you should skip to step 3. Otherwise continue with step 2.