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

Chapter 11, Configuring Off-Host Processing
Implementing Off-Host Processing Solutions
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Note If the volume was created before release 3.2 of VxVM, and it has any attached
snapshot plexes or it isassociated withany snapshotvolumes, followthe procedure
given in “Enabling Persistent FastResync on Existing Volumes with Associated
Snapshots” on page 209.
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 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 a backup, you should usually only require the default of one.
4. If the volume to be backed up contains database tables in a file system, suspend
updates to the volume. The database may have a hot backup mode that allows you to
do this by temporarily suspending writes to its tables.