VERITAS Storage Foundation 4.1 Intelligent Storage Provisioning Administrator's Guide

Creating a Shared Cache Volume and Preparing Space-Optimized Snapshots
76 VERITAS Storage Foundation ISP Administrator’s Guide
Note The command shown in this and subsequent steps assumes that you are using a
Bourne-type shell such as sh, ksh or bash. You may need to modify the command for
other shells such as csh or tcsh.
2. Use the vxvoladm command to create a volume, snapvol, of the required size and
redundancy:
# vxvoladm [-g diskgroup] [-p storage_pool] make snapvol $LEN \
[storage_specification ...] [attribute ...] type=snapshot \
[regionsize=size] init=active
The attribute regionsize specifies the minimum size of each chunk (or region) of a volume
whose contents are tracked for changes. The region size must be a 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. The default region size is 64k (64KB).
Note 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.
The init=active attribute is specified to make the volume available immediately.
The following example creates a 10-gigabyte mirrored volume, ttsnpvol, in the clone
storage pool, ttclpool:
# vxvoladm -g ttdg -p ttclpool make ttsnpvol 10g \
capability=’DataMirroring(nmirs=2)’ type=snapshot init=active
Creating a Shared Cache Volume and Preparing Space-Optimized
Snapshots
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.