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

Chapter 9, Administering Volume Snapshots
Cascaded Snapshots
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See “Creating Instant Snapshots” on page 275 for details of the procedures for creating
and using this type of snapshot.
For information about how to add snapshot mirrors to a volume, see “Adding Snapshot
Mirrors to a Volume” on page 285.
Cascaded Snapshots
A snapshot hierarchy known as a snapshot cascade can improve write performance for
some applications. Instead of having several independent snapshots of the volume, it is
more efficient to make the older snapshots into children of the latest snapshot as shown in
“Snapshot Cascade.”
Snapshot Cascade
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.
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.
Original
volume
V
Snapshot
volume
Sn
Snapshot
volume
Sn-1
Snapshot
volume
S1
. . .
Most recent
snapshot
Oldest
snapshot