Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Creating Traditional Third-Mirror Break-Off Snapshots
300 VERITAS Volume Manager Administrator’s Guide
Once the snapshot mirror is synchronized, it continues being updated until it is
detached. You can then select a convenient time at which to create a snapshot volume as
an image of the existing volume. You can also ask users to refrain from using the system
during the brief time required to perform the snapshot (typically less than a minute).
The amount of time involved in creating the snapshot mirror is long in contrast to the
brief amount of time that it takes to create the snapshot volume.
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 volume and the state of the snapshot is set to ACTIVE.
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 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 161. 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 203 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.