About determining cache size

The required size of the cache partition depends on how much user write activity occurs while the snapshot is active. The required cache size does not depend on the size of the file system. The more user activity, the larger the cache must be. You can use the snaplist and snapcachelist commands to determine the required size of the cache.

See Determining a size for the cache partition.

If a cache is too small and overflows, all snapshots using the cache become unreadable and the backups that read the snapshots fail.

See About managing nbu_snap.