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Volume snapshots, such as those described in Traditional third-mirror break-off snapshots and Full-sized instant snapshots, 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 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
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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 space-optimized snapshots, reattach them to their original volume, or turn them into independent volumes.
See Creating and managing space-optimized instant snapshots for details of the procedures for creating and using this type of snapshot.
For information about how to set up a cache for use by space-optimized instant snapshots, see Creating a shared cache object.