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Cascaded snapshots

Snapshot cascade shows a snapshot hierarchy, known as a snapshot cascade, that can improve write performance for some applications.

Snapshot cascade

Snapshot cascade

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Instead of having several independent snapshots of the volume, it is more efficient to make the older snapshots into children of the latest snapshot.

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:

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" on page 318.

  Note   Only unsynchronized full-sized or space-optimized instant snapshots are usually cascaded. It is of little utility to create cascaded snapshots if the infrontof snapshot volume is fully synchronized (as, for example, with break-off type snapshots).