VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Implementing Off-Host Processing Solutions
278 VERITAS Volume Manager Administrator’s Guide
If the volume is not associated with a DCO object and DCO volume, follow the
procedure described in “Adding a DCO and DCO Volume” on page 193.
2. Use the following command on the primary host to check whether FastResync is
enabled on a volume:
# vxprint -g volumedg -F%fastresync volume
This command returns on if FastResync is enabled; otherwise, it returns off.
If Persistent FastResync is disabled, enable it using the following command on the
master host:
# vxvol -g volumedg set fastresync=on volume
3. If the volume does not alreadycontain asnapshot plex,create one 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 onotherdisks inthedisk group.If the snapshotplexesareplaced 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 126. 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 166 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
mirrorhas beensynchronized with the volume.The mirror is then ready tobe usedas
a plex of a snapshot volume. While attached to the original volume, its contents
continue to be updated until you take the snapshot.
Use the nmirror attribute to create as many snapshot mirrors as you need for the
snapshot volume.For applications where data redundancyis required for the volume
that contains the replica database, specify a number greater than one.