Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

See “Creating a volume for use as a full-sized instant or linked break-off snapshot”
on page 217.
Cascaded snapshots
Figure 17-4 shows a snapshot hierarchy, known as a snapshot cascade, that can
improve write performance for some applications:
Figure 17-4
Snapshot cascade
Most recent
snapshot
Oldest
snapshot
Original volume
V
Snapshot volume
Sn-1
Snapshot volume
Sn
Snapshot volume
S1
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:
■ The deletion of a snapshot in the cascade copies 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.
207Administering volume snapshots
Cascaded snapshots