Volume Shadow Copy Service
| A component of Microsoft Windows | |
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Previous Versions in Windows Vista, a part of Windows Explorer that allows persistent shadow copies to be created
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| Details | |
| Other names |
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| Included with |
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| Also available for | Windows 2000, Windows XP RTM or SP1 |
| Service name | VSS |
| Description | Volume Shadow Copy |
| Related components | |
| Backup and Restore, File History | |
Shadow Copy (also known as Volume Snapshot Service,Volume Shadow Copy Service or VSS) is a technology included in Microsoft Windows that allows taking manual or automatic backup copies or snapshots of computer files or volumes, even when they are in use. It is implemented as a Windows service called the Volume Shadow Copy service. A software VSS provider service is also included as part of Windows to be used by Windows applications. Shadow Copy technology requires the file system to be NTFS in order to create and store shadow copies. Shadow Copies can be created on local and external (removable or network) volumes by any Windows component that uses this technology, such as when creating a scheduled Windows Backup or automatic System Restore point.
VSS operates at the block level of volumes.
A snapshot is a read-only point-in-time copy of the volume. Snapshots allow the creation of consistent backups of a volume, ensuring that the contents do not change and are not locked while the backup is being made.
The core component of shadow copy is the Volume Shadow Copy service, which initiates and oversees the snapshot creation process. The components that perform all the necessary data transfer are called providers. While Windows comes with a default System Provider, software and hardware vendors can create their own software or hardware providers and register them with Volume Shadow Copy service. Each provider has a maximum of 10 seconds' time to complete the snapshot generation.
Other components that are involved in the snapshot creation process are writers. The aim of Shadow Copy is to create consistent reliable snapshots. But sometimes, this cannot simply be achieved by completing all pending file change operations. Sometimes, it is necessary to complete a series of inter-related changes to several related files. For example, when a database application transfers a piece of data from one file to another, it needs to delete it from the source file and create it in the destination file. Hence, a snapshot must not be between the first deletion and the subsequent creation, or else it is worthless; it must either be before the deletion or after the creation. Enforcing this semantic consistency is the duty of writers. Each writer is application-specific and has 60 seconds to establish a backup-safe state before providers start snapshot creation. If the Volume Shadow Copy service does not receive acknowledgement of success from the corresponding writers with this time-frame, it fails the operation.
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