Shadow Copy

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Shadow Copy is a feature in recent versions of Microsoft Windows that provides periodic snapshots of a system's volumes. It was also called Volume Snapshot Service (VSS).

1 Overview
2 History
3 See also
4 References

[edit] Overview

Snapshots have two primary purposes. They allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made, avoiding problems with file locking; because the backup software is using a read-only copy of the volume, it is able to access every file without interfering with other programs writing to those same files. Microsoft's own backup utility, included in Windows XP, makes use of this mechanism. Also, users can access their files as they existed at the time of the snapshot, thus retrieving an earlier version of a file or recovering a file deleted by mistake.

The end result is similar to a versioning file system, allowing any file to be retrieved as it existed at the time any of the snapshots was made. Unlike a true versioning file system, however, users cannot trigger the creation of new versions of an individual file, only the entire volume. As a side-effect, where the owner of a file can create new versions in a versioning file system, only a system administrator or a backup operator can create new snapshots (or control when new snapshots are taken), because this requires control of the entire volume rather than an individual file. Also, many versioning file systems (such as the one in VMS) implicitly save a version of files each time they are changed; systems using a snapshotting approach like Vista only capture the state periodically.

[edit] History

The snapshot facility was first added to Microsoft Windows in Windows XP; this version could only create non-persistent snapshots (a temporary snapshot, usually used for creating a backup). The creation of persistent snapshots (multiple snapshots which remain available until specifically deleted from the system) was added in Windows Server 2003, allowing up to 512 snapshots to exist simultaneously for the same volume, from which maximum 64 snapshots could be used for the Shadow copies for Shared Folders feature. These snapshots may also be accessed over the network, although doing so on client platforms earlier than Windows Server 2003 requires the addition of client software. A copy of this software for 32-bit Windows platforms is available on the server. The client software is included in Windows XP Service Pack 2 by default; therefore, the share in the server is not needed.

In Windows Vista, this feature was renamed to Previous Versions, and was enabled by default. It is available for local files in the Business, Enterprise, and Ultimate editions.