VERITAS FlashSnap Point-In-Time-Copy Solutions Administrator's Guide

Creating a Shared Cache Object
24 VERITAS FlashSnap Point-In-Time Copy Solutions Administrator’s Guide
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 instant space-optimized 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.
2. Having decided on its characteristics, use the vxassist command to create the volume that
is to be used for 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 is specified to make the cache volume immediately available for
use.