Traditional Third-Mirror Break-Off Snapshots

The traditional third-mirror break-off volume snapshot model that is supported by the vxassist command is shown in Third-Mirror Snapshot Creation and Usage. This figure also shows the transitions that are supported by the snapback and snapclear commands to vxassist.

Third-Mirror Snapshot Creation and Usage

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The vxassist snapstart command creates a mirror to be used for the snapshot, and attaches it to the volume as a snapshot mirror. (The vxassist snapabort command can be used to cancel this operation and remove the snapshot mirror.)

Note 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. For methods of making snapshot plexes immediately available, see Full-Sized Instant Snapshots and Space-Optimized Instant Snapshots.

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 each time that you use it to make a backup. You can also use a variation of the same command to restore the contents of the original volume from a snapshot that you took at an earlier point in time. See Restoring the Original Volume from a Snapshot for more information.

As described in FastResync, you can use the FastResync feature of VxVM to minimize the time needed to resynchronize the data in the snapshot mirror. 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. The snapshot volume then has an existence that is independent of the original volume. This is useful for applications that do not require the snapshot to be resynchronized with the original volume.

Note 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. Instead, the vxsnap command may be used as described in the following section.

See Creating Traditional Third-Mirror Break-Off Snapshots for a description of the procedures for creating and using this type of snapshot.


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Product: Volume Manager Guides
Manual: Volume Manager 4.1 Administrator's Guide
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