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

Chapter 8, Administering Volumes
Backing up Volumes Online
215
The online backup procedure is completed by running the vxassist snapshot
command on a volume with a SNAPDONE mirror. This task detaches the finished
snapshot (which becomes a normal mirror), creates a new normal volume and attaches
the snapshot mirror to the snapshot volume. The snapshot then becomes a normal,
functioning mirror and the state of the snapshot is set to ACTIVE.
If the snapshot procedure is interrupted, the snapshot mirror is automatically
removed when the volume is started.
To back up a volume with the vxassist command, use the following procedure:
1. Create a snapshot mirror for a volume using the following command:
# vxassist [-b] [-g diskgroup] snapstart [nmirror=N] volume
For example, to create a snapshot mirror of a volume called voldef, use the
following command:
# vxassist [-g diskgroup] snapstart voldef
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 diskgroup] 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.
It is also possible to make a snapshot plex from an existing plex in a volume. See
“Converting a Plex into a Snapshot Plex” on page 217 for details.