Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 9, Administering Volume Snapshots
Creating Instant Snapshots
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power of 2, and be greater than or equal to 16KB. A smaller value requires more disk
space for the change maps, but the finer granularity provides faster
resynchronization.
For space-optimized instant snapshots that share a cache object, the specified region
size must be greater than or equal to the region size specified for the cache object. See
“Creating a Shared Cache Object” on page 294 for details.
Note Creation of space-optimized snapshots that use a shared cache fails if the region
size specified for the volume is smaller than the region size set on the cache.
If the region size of a space-optimized snapshot differs from the region size of
the cache, this can degrade the system’s performance compared to the case
where the region sizes are the same.
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 416), 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).
vxsnap make operations to create full-sized or space-optimized instant snapshots,
and refresh and restore operations on such volumes fail as they might
potentially hang the system. Such volumes can be used only for breakoff-type
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
3. For full-sized instant snapshots, you must create a volume that is to be used as the
snapshot volume. This must be the same size as the volume for which the snapshot is
being created, and it must also have the same region size. See “Creating a Volume for
Use as a Full-Sized Instant Snapshot” on page 293 for details.
Alternatively, if there is a sufficient number of suitable plexes available in the volume,
you can break these off and use them to create a full-sized instant snapshot. You can
either use existing ACTIVE plexes in the volume, or you can use the following
command to add new snapshot mirrors to the volume: