Volume Shadow Copy Service

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The Volume Shadow Copy Service, also called Volume Snapshot Service (VSS), in modern versions of the Microsoft Windows operating system provides periodic snapshots of a system's volumes.

[edit] Overview

Snapshots have two primary purposes: first, they allow the creation of consistent backups of a volume, ensuring that the contents cannot change while the backup is being made, as well as 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 with Windows XP, makes use of this mechanism.

Secondly, 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.

The snapshot facility was first added to Microsoft Windows in Windows XP, although 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 then 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 as %SYSTEMROOT%\system32\clients\twclient\x86\twcli32.msi on the server.

Note: The client software comes default with Windows XP Service Pack 2. As such the share in 2003 server is not needed.

In Windows Vista, this feature is titled "Previous Versions", and is enabled by default on new installations.