Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

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If you need several space-optimized instant snapshots for the volumes in a
disk group, you may find it convenient to create a single shared cache object
in the disk group rather than a separate cache object for each snapshot.
See “Creating a shared cache object” on page 368.
For full-sized instant snapshots and linked break-off snapshots, you must
prepare a volume that is to be used as the snapshot volume. This volume must
be the same size as the data 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 or linked break-off
snapshot” on page 370.
Creating a shared cache object
To create a shared cache object
1
Decide on the following characteristics that you want to allocate to the cache
volume that underlies the cache object:
■ The cache volume size 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.
■ The cache volume can be mirrored for redundancy.
■ 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 not be shared with disks used
by critical volumes 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. The following example creates a mirrored cache volume,
cachevol, with size 1GB in the disk group, mydg, on the disks mydg16 and
mydg17:
# vxassist -g mydg make cachevol 1g layout=mirror \
init=active mydg16 mydg17
The attribute init=active makes the cache volume immediately available
for use.
Administering volume snapshots
Creating instant snapshots
368