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313Administering volume snapshots
Cascaded snapshots
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” on page 337 for an
example of the use of the
infrontof attribute.
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).
Creating a snapshot of a snapshot
For some applications, it may be desirable to create a snapshot of an existing
snapshot as illustrated in Figure 9-5.