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

Chapter 9, Administering Volume Snapshots
Space-Optimized Instant Snapshots
267
Space-Optimized Instant Snapshots
Volume snapshots, such as those described in “Traditional Third-Mirror Break-Off
Snapshots” on page 263 and “Full-Sized Instant Snapshots” on page 265, 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 294 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 “Space-Optimized Instant
Snapshot Creation and Usage in a Backup Cycle.”
Space-Optimized Instant Snapshot Creation and Usage in a Backup Cycle
Backup
Cycle
Original
volume
Back up to disk, tape or other
media.
Snapshot
volume
START
vxsnap make
vxsnap prepare
vxsnap refresh