Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

338 Administering volume snapshots
Creating traditional third-mirror break-off snapshots
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 volume and the state of the snapshot is set to ACTIVE.
To back up a volume using the vxassist command
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 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 “Moving DCO volumes between disk groups” on page 192. 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 236 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
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.
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 340 for details.
2 Choose a suitable time to create a snapshot. If possible, plan to take the snapshot at a
time when users are accessing the volume as little as possible.
3 Create a snapshot volume using the following command:
# vxassist [-g diskgroup] snapshot [nmirror=N] volume snapshot
If required, use the nmirror attribute to specify the number of mirrors in the
snapshot volume.