Snapshot (computer storage)

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In computer file systems, a snapshot is a copy of a set of files and directories as they were at a particular point in the past. Snapshots are useful for avoiding version skew when backing up volatile data sets, such as tables in a busy database or the folder store of a busy mail server.

Some file systems, such as WAFL, fossil for Plan 9 from Bell Labs or ODS-5, internally track old versions of files and make snapshots available through a special namespace. Others, like NTFS or UFS2, provide an operating system API for accessing file histories.

Some Unix systems (including Linux and HP-UX) may also have snapshot-capable logical volume managers. These implement copy-on-write on entire block devices by copying changed blocks—just before they are to be overwritten—to other storage, thus preserving a self-consistent past image of the block device. Filesystems on this image can later be mounted as if it were on read-only media. Block-level snapshotting is almost always less space-efficient than direct file system support for snapshots.

Read-write snapshots are sometimes called branching snapshots, because they implicitly create diverging versions of their data.

Shadow paging and write ahead logging are similar snapshot-like mechanisms used internally by many databases to implement transactions.

The concept of a snapshot can also be applied to data structures held only in memory, for example in the implementation of software transactional memory. A "version" of a persistent data structure is effectively a snapshot.

Some backup software provide this service as a seperated option. For example, in Backup Exec, it is called Open File Option (OFO).

Microsoft provided a similar service in system level, called Volume Shadow Copy Service (VSS) in Windows XP and Windows Server 2003.