Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Cascaded Snapshots

270 VERITAS Volume Manager Administrator’s Guide

◆ 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 286 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 “Creating a Snapshot of a Snapshot.”

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 269, 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

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. These steps are illustrated in “Using a Snapshot of a Snapshot to Restore a

Database” on page 271.

vxsnap make source=V vxsnap make source=S1

Original

volume

Snapshot

volume

Snapshot

volume