VERITAS Volume Manager 4.1 Administrator's Guide

Administering Volumes
Backing up Volumes Online
Chapter 8310
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:
Step 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 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 177. 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