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.

In contrast, 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.

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: Space-optimized instant snapshot creation and usage in a backup cycle shows the instant space-optimized snapshot model.

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.

A cache object and cache volume must be set up for use by space-optimized instant snapshots.