| Volume Shadow Copy Service |
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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 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 64 snapshots to exist simultaneously. 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 installed as %SYSTEMROOT%\TWCLIENT†\TWCLI32.MSI on the server. With that software installed, 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 present was made. Unlike a true versioning file system, however, you cannot create 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 can create new snapshots (or control when new snapshots are taken), because this requires control of the entire volume rather than an individual file. |
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