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

304 Administering volume snapshots
Cascaded snapshots
is complete. The vxsnap snapwait command can be used to wait for the state to become
ACTIVE.
.When you use the vxsnap make command to create the snapshot volume, this removes
the link, and establishes a snapshot relationship between the snapshot volume and the
original volume.
The
vxsnap reattach operation re-establishes the link relationship between the two
volumes, and starts a resynchronization of the mirror volume.
See “Creating and managing linked break-off snapshot volumes” on page 321 for details
of the procedures for creating and using this type of snapshot.
For information about how to prepare an empty volume for use by full-sized instant
snapshots, see “Creating a volume for use as a full-sized instant or linked break-off
snapshot” on page 314.
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
Figure 9-4.
Figure 9-4 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.
Oldest
snapshot
Most recent
snapshot
Original
volume
V
Snapshot
volume
Sn
Snapshot
volume
Sn-1
Snapshot
volume
S1
. . .