Administering volume snapshots 349

Creating traditional third-mirror break-off snapshots

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 using the vxassist command

1Create 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 200. 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 244 for more information.

If you start vxassist snapstart in the background using the -boption, 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.