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

301Administering volume snapshots
Space-optimized instant snapshots
Space-optimized instant snapshots
Volume snapshots, such as those described in “Traditional third-mirror break-off
snapshots” on page 297 and “Full-sized instant snapshots” on page 299, require the
creation of a complete copy of the original volume, and use as much storage space as the
original volume.
Instead of requiring a complete copy of the original volume’s storage space, space-
optimized instant snapshots use a storage cache. The size of this cache may be configured
when the snapshot is created.
Note: A storage cache may be named and shared among several volumes in the same disk
group. If so, the size of the cache that is declared must be the same for each volume’s
space-optimized snapshot. You may find it convenient to configure a single storage cache
in a disk group that can be shared by all the volumes in that disk group. See “Creating a
shared cache object” on page 312 for details.
When the original volume is written to, VxVM preserves the original data contents in the
cache before the write is committed. As the storage cache can be configured to require
much less storage than the original volume, it is referred to as being space-optimized. If
the cache becomes too full, you can configure VxVM to grow the size of the cache
automatically using any available free space in the disk group.
The instant space-optimized snapshot model is illustrated in Figure 9-3.
Figure 9-3 Space-optimized instant snapshot creation and usage in a backup cycle
As for instant snapshots, space-optimized snapshots use a copy-on-write mechanism to
make them immediately available for use when they are first created, or when their data is
refreshed. Unlike instant snapshots, however, you cannot enable synchronization on
Backup
Cycle
Original
volume
Back up to disk, tape or
other media.
Snapshot
volume
Start
vxsnap make
vxsnap prepare
vxsnap refresh