Creating traditional third-mirror break-off snapshots

VxVM provides third-mirror break-off snapshot images of volume devices using vxassist and other commands.

To enhance the efficiency and usability of volume snapshots, turn on FastResync.

If Persistent FastResync is required, you must associate a version 0 DCO with the volume.

A plex is required that is large enough to store the complete contents of the volume. Alternatively, you can use space-optimized instant snapshots.

The recommended approach to performing volume backup from the command line, or from a script, is to use the vxsnap command. The vxassist snapstart, snapwait, and snapshot commands are supported for backward compatibility.

The vxassist snapshot procedure consists of two steps:

The vxassist snapstart step creates a write-only backup plex which gets attached to and synchronized with the volume. When synchronized with the volume, the backup plex is ready to be used as a snapshot mirror. The end of the update procedure is indicated by the new snapshot mirror changing its state to SNAPDONE. This change can be tracked by the vxassist snapwait task, which waits until at least one of the mirrors changes its state to SNAPDONE. If the attach process fails, the snapshot mirror is removed and its space is released.


If the snapstart procedure is interrupted, the snapshot mirror is automatically removed when the volume is started.

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 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. Clean the temporary volume's contents using an appropriate utility such as fsck for non-VxVM file systems and log replay for databases. Because VxVM calls VxFS and places VxFS file systems in a constant state immediately before taking a snapshot, it is not usually necessary to run fsck on a VxFS file system on the temporary volume. If a VxFS file system contains a database, it will still be necessary to perform database log replay.
  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.

More Information

Adding a version 0 DCO and DCO volume

Moving DCO volumes between disk groups

Creating a volume on specific disks

Converting a plex into a snapshot plex

Creating multiple snapshots with the vxassist command

Reattaching a snapshot volume

Dissociating a snapshot volume