Figure: Snapshot cascade shows a snapshot hierarchy, known as a snapshot cascade, that 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.
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.
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.
The following points determine whether it is appropriate 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.