About sizing the cache for Instant Recovery copy-on-write snapshots

A copy-on-write snapshot requires cache space for storing the changes that occur on the source device during the life of the snapshot. While the snapshot is active, any blocks that are about to be changed by user activity are copied to the cache. Blocks that do not change on the source are not copied. Compared to a fully-allocated snapshot (clone or mirror), a copy-on-write snapshot may consume relatively little disk space and can be executed very quickly.

As a rule, the appropriate size for the cache depends on the amount of user activity that occurs during the life of the snapshot. The more changes in the source data, or the longer the life of the snapshot, the more blocks that are likely to be changed. As a result, more data must be stored in the cache.

The size of the file system or raw partition does not determine cache size. If little change activity occurs on the source during the life of the snapshot, little cache space is required, even for a large file system.