VSS is Volume Shadow Copy Service

The Volume Shadow Copy Service (VSS) is a set of COM APIs that implements a framework to allow volume backups to be performed while applications on a system continue to write to the volumes. It provides the backup infrastructure as well as a mechanism for creating consistent point-in-time copies of data known as shadow copies. It can produce consistent shadow copies by coordinating with business applications, file-system services, backup applications, fast-recovery solutions, and storage hardware. VSS provides a consistent interface that allows coordination between user applications that update data on disk (writers) and those that back up applications (requesters).

The Volume Shadow Copy Service (VSS) captures and copies stable images for backup on running systems, particularly servers, without unduly degrading the performance and stability of the services they provide. The VSS service starts on demand; therefore, for VSS operations to be successful, this service must be enabled.

Though largely transparent to user and developer, VSS does the following:

•  Coordinates activities of providers, writers, and requesters in the creation and use of shadow copies.
•  Furnishes the default system provider.
•  Implements low-level driver functionality necessary for any provider to work.

There are two methods for creating shadow copies: making either a complete copy (a full copy or clone) or copying only the changes to the volume (a differential copy or copy-on-write). Each method results in two data images - the original volume and the shadow copy volume. The functional difference between the two is that the original volume maintains full read/write capabilities, whereas the shadow copy volume is read-only. This read-only status ensures that the shadow copy volume remains a point-in-time copy until its status is changed by the administrator for a specific purpose.

Keywords of VSS:
Volume Shadow Copy Service	A service that coordinates various components to create consistent shadow copies of one or more volumes.
Requestor	An application that requests that a volume shadow copy be taken. A backup application is an example.
Writer	A component of an application that stores persistent information on one or more volumes that participate in shadow copy synchronization. Typically, this is a database application like SQL Server or Exchange Server, or a system service like Active Directory.
Provider	A component that creates and maintains the shadow copies. Examples are the system provider included with the operating system and the hardware providers included with storage arrays.
Source volume	The volume that contains the data to be shadow copied.
Storage volume	The volume that holds the shadow copy storage files for the system copy-on-write software provider.