Cascaded Snapshots

A snapshot hierarchy known as a snapshot cascade can improve write performance for some applications. Instead of having several independent snapshots of the volume, it is more efficient to make the older snapshots into children of the latest snapshot as shown in Snapshot Cascade.

Snapshot Cascade

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A snapshot may be added to a cascade by specifying the infrontof attribute to the vxsnap make command when the second and subsequent snapshots in the cascade are created. Changes to blocks in the original volume are only written to the most recently created snapshot volume in the cascade. If an attempt is made to read data from an older snapshot that does not exist in that snapshot, it is obtained by searching recursively up the hierarchy of more recent snapshots.

A snapshot cascade is most likely to be used for regular online backup of a volume where space-optimized snapshots are written to disk but not to tape.

A snapshot cascade improves write performance over the alternative of several independent snapshots, and also requires less disk space if the snapshots are space-optimized. Only the latest snapshot needs to be updated when the original volume is updated. If and when required, the older snapshots can obtain the changed data from the most recent snapshot.

The following points determine whether it is appropriate for an application to use a snapshot cascade:

• Deletion of a snapshot in the cascade takes time to copy the snapshot's data to the next snapshot in the cascade.
• The reliability of a snapshot in the cascade depends on all the newer snapshots in the chain. Thus the oldest snapshot in the cascade is the most vulnerable.
• Reading from a snapshot in the cascade may require data to be fetched from one or more other snapshots in the cascade.

For these reasons, it is recommended that you do not attempt to use a snapshot cascade with applications that need to remove or split snapshots from the cascade. In such cases, it may be more appropriate to create a snapshot of a snapshot as described in the following section.

See Adding a Snapshot to a Cascaded Snapshot Hierarchy for an example of the use of the infrontof attribute.

Creating a Snapshot of a Snapshot

For some applications, it may be desirable to create a snapshot of an existing snapshot as illustrated in Creating a Snapshot of a Snapshot.

Creating a Snapshot of a Snapshot

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Even though the arrangement of the snapshots in this figure appears similar to the snapshot hierarchy shown in Snapshot Cascade, the relationship between the snapshots is not recursive. When reading from the snapshot S2, data is obtained directly from the original volume, V, if it does not exist in S2 itself.

Such an arrangement may be useful if the snapshot volume, S1, is critical to the operation. For example, S1 could be used as a stable copy of the original volume, V. The additional snapshot volume, S2, can be used to restore the original volume if that volume becomes corrupted. For a database, you might need to replay a redo log on S2 before you could use it to restore V. These steps are illustrated in Using a Snapshot of a Snapshot to Restore a Database.

Using a Snapshot of a Snapshot to Restore a Database

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If you have configured snapshots in this way, you may wish to make one or more of the snapshots into independent volumes. There are two vxsnap commands that you can use to do this:

• vxsnap dis dissociates a snapshot volume and turns it into an independent volume. The volume to be dissociated must have been fully synchronized from its parent. If a snapshot volume has a child snapshot volume, the child must also have been fully synchronized. If the command succeeds, the child snapshot becomes a snapshot of the original volume. Dissociating a Snapshot Volume illustrates the effect of applying this command to snapshots with and without dependent snapshots.

Dissociating a Snapshot Volume



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• vxsnap split dissociates a snapshot and its dependent snapshots from its parent volume. The snapshot volume that is to be split must have been fully synchronized from its parent volume. This operation is illustrated in Splitting Snapshots.

Splitting Snapshots



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