Veritas Storage Foundation Intelligent Storage Provisioning 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

97Administering instant snapshots
Creating a shared cache volume and preparing space-optimized snapshots
Creating a shared cache volume and preparing
space-optimized snapshots
Note: If you intend to create space-optimized instant snapshots that share a
cache volume, the region size that you specify for the volume must be greater
than or equal to any region size that you specify for the cache volume. Creation
of space-optimized snapshots that use a shared cache fails if the region size of
the volume is smaller than the region size of 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.
If you need to create several space-optimized instant snapshots for the volumes
in a disk group, you may find it more convenient to create a single shared cache
volume in the disk group rather than a separate cache volume for each
snapshot.
To create a shared cache volume and prepare any space-optimized snapshots
1 Decide on the following characteristics that you want to allocate to the cache
volume:
■ The size of the cache volume should be sufficient to record changes to
the parent volumes during the interval between snapshot refreshes. A
suggested value is 10% of the total size of the parent volumes for a
refresh interval of 24 hours.
■ If redundancy is a desired characteristic of the cache volume, it should
be mirrored. This increases the space required by the cache volume in
proportion to the number of mirrors that it has.
■ If the cache volume is mirrored, space is required on at least as many
disks as it has mirrors. These disks should not be shared with the disks
used for the parent volumes. The disks should also be chosen to avoid
impacting I/O performance for critical volumes, or hindering disk
group split and join operations.
2 Having decided on its characteristics, use the
vxassist command to create
the cache volume:
# vxassist [-g diskgroup ] [-P storage_pool ] make cachevol \
size [ storage_specification ...] [ attribute ...] \
[regionsize
size ] type=cachevolume
The attribute regionsize specifies the minimum size of each chunk (or
region) of a volume whose contents are tracked for changes. The region size