A storage snapshot is a set of reference markers, or pointers, to data stored on a disk drive, on a tape, or in a storage area network (SAN). A snapshot is something like a detailed table of contents, but it is treated by the computer as a complete data backup. Snapshots streamline access to stored data and can speed up the process of data recovery. There are two main types of storage snapshot, called the copy-on-write (or low-capacity) snapshot and the split-mirror snapshot. Utilities are available that can automatically generate either type.
A copy-on-write snapshot utility creates a snapshot of changes to stored data every time new data is entered or existing data is updated. This allows rapid recovery of data in case of a disk write error, corrupted file, or program malfunction. However, all previous snapshots must be available if complete archiving or recovery of all the data on a network or storage medium is needed.
A split-mirror snapshot utility references all the data on a set of mirrored drives. Every time the utility is run, a snapshot is created of the entire volume, not only of the new or updated data. This makes it possible to access data offline, and simplifies the process of recovering, duplicating, or archiving all the data on a drive. However, this is a slower process, and it requires more storage space for each snapshot.