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

Creating Instant Snapshots
276 VERITAS Volume Manager Administrator’s Guide
To back up a volume with the vxsnap command, use the following procedure:
1. If you intend to take a space-optimized instant snapshot of the volume, you may wish
to consider first setting up a shared cache object in the same disk group as the volume.
This cache object can be maintained by using the vxcache command.
See “Creating a Shared Cache Object” on page 294 for details of how to set up a
shared cache object.
Note If you intend to split the volume and snapshot into separate disk groups (for
example, to perform off-host processing), you must use a fully synchronized
full-sized instant snapshot (which does not require a cache object). You cannot
use a space-optimized instant snapshot for this purpose.
2. Use the following commands on the primary host to see if the volume is associated
with a version 20 data change object (DCO) and DCO volume that allow instant
snapshots and Persistent FastResync to be used with the volume, and to check that
FastResync is enabled on the volume:
# vxprint -g volumedg -F%instant volume
# vxprint -g volumedg -F%fastresync volume
If both these command return a value of on, the volume can be used for instant
snapshot operations.
To prepare a volume for instant snapshots, use the following command:
# vxsnap [-g diskgroup] prepare volume [regionsize=size] \
[ndcomirs=number] [alloc=storage_attributes]
Note It is only necessary to run the vxsnap prepare command on a volume if it
does not already have a version 20 DCO volume (for example, if you have run
the vxsnap unprepare command on the volume). See “Creating a Volume
with a Version 20 DCO Volume” on page 212, “Preparing a Volume for DRL and
Instant Snapshots” on page 235 and “Removing Support for DRL and Instant
Snapshots from a Volume” on page 239 for more information.
For example, to prepare the volume, myvol, in the disk group, mydg, use the
following command:
# vxsnap -g mydg prepare myvol regionsize=128k ndcomirs=2 \
alloc=mydg10,mydg11
This example creates a DCO object and redundant DCO volume with two plexes
located on disks mydg10 and mydg11, and associates them with myvol. The region
size is also increased to 128KB from the default size of 64KB. The region size must be a