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

Creating a Shared Cache Object
294 VERITAS Volume Manager Administrator’s Guide
4. Use the vxassist command to create a volume, snapvol, of the required size and
redundancy, together with a version 20 DCO volume with the correct region size:
# vxassist [-g diskgroup] make snapvol $LEN \
[layout=mirror nmirror=number] logtype=dco drl=off \
dcoversion=20 [ndcomirror=number] regionsz=$RSZ init=active \
[storage_attributes]
Specify the same number of DCO mirrors (ndcomirror) as the number of mirrors in
the volume (nmirror). The init=active attribute is used to make the volume
available immediately. You can use storage attributes to specify which disks should be
used for the volume.
As an alternative to creating the snapshot volume and its DCO volume in a single
step, you can first create the volume, and then prepare it for instant snapshot
operations as shown here:
# vxassist [-g diskgroup] make snapvol $LEN \
[layout=mirror nmirror=number] init=active [storage_attributes]
# vxsnap [-g diskgroup] prepare snapvol [ndcomirs=number] \
regionsize=$RSZ [storage_attributes]
Creating a Shared Cache Object
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 object in the disk
group rather than a separate cache object for each snapshot.
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 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 that is required for 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.