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

Space-optimized instant snapshots
Volume snapshots require the creation of a complete copy of the original volume,
and use as much storage space as the copy of the original volume.
Space-optimized instant snapshots do not require a complete copy of the original
volume’s storage space. They use a storage cache.
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. If so, the size of the cache
that is declared must be the same for each volume’s space-optimized snapshot.
The cache is stored on disk and is persistent.
See “Creating a shared cache object” on page 368.
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 typically requires
much less storage than the original volume, it is referred to as space-optimized.
If the cache approaches full, you can configure VxVM to grow the cache
automatically using any available free space in the disk group.
Figure 9-3 shows the instant space-optimized snapshot model.
Figure 9-3
Space-optimized instant snapshot creation and usage in a backup
cycle
Start
Original volume
vxsnap prepare
vxsnap make
vxsnap refresh
Backup
cycle
Snapshot volume
Back up to disk, tape or
other media
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, you cannot enable synchronization on
space-optimized snapshots, reattach them to their original volume, or turn them
into independent volumes.
See “Creating and managing space-optimized instant snapshots” on page 371.
A cache object and cache volume must be set up for use by space-optimized instant
snapshots.
See “Creating a shared cache object” on page 368.
Administering volume snapshots
Space-optimized instant snapshots
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