Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

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 384.
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
Figure 9-5 creation of a snapshot of an existing snapshot.
Figure 9-5
Creating a snapshot of a snapshot
vxsnap make source=V vxsnap make source=S1
Original volume
V
Snapshot volume
S1
Snapshot volume
S2
Even though the arrangement of the snapshots in this figure appears similar to
a snapshot cascade, 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.
See Figure 9-4 on page 359.
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 snapshot volume, S2, can be used to restore the original volume
if that volume becomes corrupted. For a database, you might need to replay a redo
log on S2 before you could use it to restore V.
Figure 9-6 shows the sequence of steps that would be required to restore a database.
Administering volume snapshots
Cascaded snapshots
360