Microsoft Volume Shadow Copy Service (Microsoft VSS), is a Microsoft Windows Server feature that takes snapshots of volumes that can be used for restoring logical unit numbers and individual files or for data mining. The snapshots can be taken manually or automatically.
VSS was introduced in Windows Server 2003. It allows application data to be backed up while the applications are still running. It is compatible with NTFS. The feature can be managed as a hardware provider or software provider. A hardware provider manages shadow copies in conjunction with a hardware storage controller and the software provider intercepts input/output requests between the file system and volume manager.
There are several methods by which Volume Shadow Copy Service can make copies. The first is a complete copy, which creates a read-only copy of the original volume. The second method is copy-on-write, which copies only the changes to a volume after a given point in time. It can also make a redirect-on-write copy, which creates a differential copy similar to that of the copy-on-write method, but redirects the changes to a different volume.