Storage Foundation 7.3 Administrator's Guide - AIX

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.

   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.

   To override the default storage allocation policy, you can use storage attributes to specify explicitly which disks to use for the snapshot plexes.

   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.

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.

   For example, to create a snapshot of voldef, use the following command:

   # vxassist -g mydg snapshot voldef snapvoldef

   The vxassist snapshot task detaches the finished snapshot mirror, creates a new volume, and attaches the snapshot mirror to it. This step should only take a few minutes. The snapshot volume, which reflects the original volume at the time of the snapshot, is now available for backing up, while the original volume continues to be available for applications and users.

   If required, you can make snapshot volumes for several volumes in a disk group at the same time.

4. Use fsck (or some utility appropriate for the application running on the volume) to clean the temporary volume's contents. For example, you can use this command with a VxFS file system:

   # fsck -V vxfs /dev/vx/dsk/diskgroup/snapshot

   The specified device must have a valid entry in the /etc/filesystems file.

5. If you require a backup of the data in the snapshot, use an appropriate utility or operating system command to copy the contents of the snapshot to tape, or to some other backup medium.

6. When the backup is complete, you have the following choices for what to do with the snapshot volume:

   • Reattach some or all of the plexes of the snapshot volume with the original volume.

   • If FastResync was enabled on the volume before the snapshot was taken, this speeds resynchronization of the snapshot plexes before the backup cycle starts again at step 3.

   • Dissociate the snapshot volume entirely from the original volume

   • This may be useful if you want to use the copy for other purposes such as testing or report generation.

   • Remove the snapshot volume to save space with this command:

     # vxedit [-g diskgroup] -rf rm snapshot

   Dissociating or removing the snapshot volume loses the advantage of fast resynchronization if FastResync was enabled. If there are no further snapshot plexes available, any subsequent snapshots that you take require another complete copy of the original volume to be made.