Editing Shadow Copy (section)

==Overview==
VSS operates at the [[Block (data storage)|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 [[file locking|locked]] while the backup is being made.

The core component of shadow copy is the Volume Shadow Copy [[Windows service|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.<ref name=How-TechNet>{{cite web |url=https://technet.microsoft.com/en-us/library/cc785914(WS.10).aspx |title=How Volume Shadow Copy Service Works |work=[[Microsoft TechNet|TechNet]] |publisher=Microsoft |date=28 March 2003 |access-date=4 January 2011 }}</ref>

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 within this time-frame, it fails the operation.<ref name=How-TechNet />

By default, snapshots are temporary; they do not survive a reboot. The ability to create persistent snapshots was added in Windows Server 2003 onward. Windows 8 removed the GUI portion necessary to browse them, but it was restored in later Windows versions. ({{section link||History}})

Windows software and services that support VSS include [[Windows Server Failover Clustering|Windows Failover Cluster]],<ref>{{cite web|url=https://technet.microsoft.com/en-us/library/cc770625(v=ws.10).aspx|title=What's New in Failover Clusters in Windows Server 2008|last=Archiveddocs|website=technet.microsoft.com|access-date=18 March 2018}}</ref> Windows Server Backup,<ref>{{Cite web|url=https://docs.microsoft.com/en-us/windows-server/storage/file-server/volume-shadow-copy-service|title=Volume Shadow Copy Service|last=JasonGerend|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> [[Hyper-V]],<ref>{{Cite web|url=https://docs.microsoft.com/en-us/virtualization/hyper-v-on-windows/reference/integration-services|title=Hyper-V Integration Services|last=scooley|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> [[Microsoft Virtual Server|Virtual Server]],<ref>{{Cite web|url=https://docs.microsoft.com/en-us/virtualization/community/team-blog/2007/20070716-microsoft-virtualization-and-virtual-server-2005-r2-sp1|title=Microsoft Virtualization and Virtual Server 2005 R2 SP1|last=scooley|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> [[Active Directory]],<ref>{{Cite web|url=https://docs.microsoft.com/en-us/windows/win32/vss/vss-backup-and-restore-of-the-active-directory|title=VSS Backup and Restore of the Active Directory - Windows applications|last=mcleanbyron|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> [[Microsoft SQL Server|SQL Server]],<ref>{{Cite web|url=https://docs.microsoft.com/en-us/biztalk/core/sql-server-database-mirroring-volume-shadow-copy-service-and-alwayson|title=SQL Server database mirroring, Volume Shadow Copy service and AlwaysOn - BizTalk Server|last=MandiOhlinger|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> [[Microsoft Exchange Server|Exchange Server]]<ref>{{Cite web|url=https://docs.microsoft.com/en-us/exchange/high-availability/disaster-recovery/disaster-recovery|title=Exchange Server data protection, Exchange disaster recovery, Exchange backup, Exchange VSS Writer, VSS Backup Exchange, Exchange Server data recovery, Exchange data recovery|last=msdmaguire|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref> and [[Microsoft SharePoint|SharePoint]].<ref>{{Cite web|url=https://docs.microsoft.com/en-us/sharepoint/dev/general-development/how-to-back-up-and-restore-a-search-service-application-in-sharepoint-using|title=Back up and restore a search service application in SharePoint using VSS|last=spdevdocs|website=docs.microsoft.com|language=en-us|access-date=2019-08-11}}</ref>

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, whereas 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. Also, many versioning file systems (such as the one in [[OpenVMS|VMS]]) implicitly save a version of files each time they are changed; systems using a snapshotting approach like Windows only capture the state periodically.