VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Configuring Off-Host Processing
Implementing Off-Host Processing Solutions
Chapter 11 377
NOTE If the volume was created before release 3.2 of VxVM, and it has any
attached snapshot plexes or it is associated with any snapshot volumes,
follow the procedure given in “Enabling Persistent FastResync on
Existing Volumes with Associated Snapshots” on page 288.
Step 3. If the volume does not already contain a snapshot plex, create a snapshot
mirror for a volume using the following command on the primary host:
# vxassist -g volumedg [-b] snapstart [nmirror= N ] volume
The vxassist snapstart task creates a write-only mirror, which is
attached to and synchronized from the volume to be backed up.
NOTE By default, VxVM attempts to avoid placing a snapshot mirrors on a disk
that already holds any plexes of a data volume. However, this may be
impossible if insufficient space is available in the disk group. In this case,
VxVM uses any available space on other disks in the disk group. If the
snapshot plexes are placed on disks which are used to hold the plexes of
other volumes, this may cause problems when you subsequently attempt
to move a snapshot volume into another disk group as described in
“Considerations for Placing DCO Plexes” on page 157. To override the
default storage allocation policy, you can use storage attributes to specify
explicitly which disks to use for the snapshot plexes. See “Creating a
Volume on Specific Disks” on page 222 for more information.
If you start vxassist snapstart in the background using the -b option, you
can use the vxassist snapwait command to wait for the creation of the
mirror to complete as shown here:
# vxassist -g volumedg snapwait volume
If vxassist snapstart is not run in the background, it does not exit until
the mirror has been synchronized with the volume. The mirror is then
ready to be used as a plex of a snapshot volume. While attached to the
original volume, its contents continue to be updated until you take the
snapshot.