Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

305Administering volume snapshots
Cascaded snapshots
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 326 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.
Figure 9-5 Creating a snapshot of a snapshot
Even though the arrangement of the snapshots in this figure appears similar to the
snapshot hierarchy shown in “Snapshot cascade” on page 304, 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
vxsnap make source=V
vxsnap make source=S1
Original
volume
V
Snapshot
volume
S1
Snapshot
volume
S2