How traditional third-mirror break-off snapshots work

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 use of the vxassist command to administer traditional (third-mirror break-off) snapshots is not supported for volumes that are prepared for instant snapshot creation. Use the vxsnap command instead.

Figure: Third-mirror snapshot creation and usage shows the traditional third-mirror break-off volume snapshot model that is supported by the vxassist command.

Figure: Third-mirror snapshot creation and usage

Third-mirror snapshot creation and usage

The vxassist snapstart command creates a mirror to be used for the snapshot, and attaches it to the volume as a snapshot mirror. As is usual when creating a mirror, the process of copying the volume's contents to the new snapshot plexes can take some time to complete. (The vxassist snapabort command cancels this operation and removes the snapshot mirror.)

When the attachment is complete, the vxassist snapshot command is used to create a new snapshot volume by taking one or more snapshot mirrors to use as its data plexes. The snapshot volume contains a copy of the original volume's data at the time that you took the snapshot. If more than one snapshot mirror is used, the snapshot volume is itself mirrored.

The command, vxassist snapback, can be used to return snapshot plexes to the original volume from which they were snapped, and to resynchronize the data in the snapshot mirrors from the data in the original volume. This enables you to refresh the data in a snapshot after you use it to make a backup. You can use a variation of the same command to restore the contents of the original volume from a snapshot previously taken.

The FastResync feature minimizes the time and I/O needed to resynchronize the data in the snapshot. If FastResync is not enabled, a full resynchronization of the data is required.

Finally, you can use the vxassist snapclear command to break the association between the original volume and the snapshot volume. Because the snapshot relationship is broken, no change tracking occurs. Use this command if you do not need to reuse the snapshot volume to create a new point-in-time copy.