VSS (Volume Shadow Copy Service)

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Shadow Copy
A component of Microsoft Windows
File:Previous Versions Vista.png
Previous Versions in Windows Vista, a part of Windows Explorer that allows persistent shadow copies to be created
Details
Other names
  • Volume Snapshot Service[1]
  • Previous Versions
  • Shadow Copies for Shared Folders8
  • VSS[2]
Included with
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.[3]

Overview

Volume Shadow Copy Service provides developers and administrators with a range of options for data backup and further data recovery on NTFS-volumes (FAT-volume is ignored). Through an open API , any developer can create their own application running with the copy. In order to implement this , two main abstractions , which operate VSS: Writer - it is an additional service that allows the image creation.

Requestor - it is the actual application that checks the integrity of the backup data and creates the necessary number of writers. VSS operates at the block level of volumes.[4]

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 can 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.[5]

By default, snapshots are temporary; they do not survive a reboot. The ability to create persistent snapshots was added in Windows Server 2003 onward. However, Windows 8 removed the GUI portion necessary to browse them. (See § History)

Windows software and services that support VSS include Windows Backup, Hyper-V, Virtual Server, Active Directory, SQL Server, Exchange Server and SharePoint.[citation needed]

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 VMS) implicitly save a version of files each time they are changed; systems using a snapshotting approach like Windows only capture the state periodically.

The implementation of both varies to each application and manages the challenges. The majority of standard applications in Windows have their own writers.

Principle of operation

The main objective of the service is to copy the information or some of the parts of it without blocking the already running applications. In fact , the access is still restricted , but it can be approached through the transaction log.

VSS.jpg

The transaction log

To ensure the integrity of the data backup , VSS is still temporarily suspend an access to the copied data applications. In order to ensure the implementation occurring during the copy creation , the real-time service records all transactions in the “journal" , delaying the implementation. The control then goes from the writer to the provider , which allows data archiving. As soon as a provider has completed the copying process , VSS (the writer) will record the transaction contents and it will apply the appropriate changes.

Applications in Windows

In addition the Windows itself often operates as a requestor , creating a recovery point of the entire system or just a separate system block. In different versions of Microsoft operating system , a different time interval is provided , after which the VSS will automatically create an image. Nevertheless , there are other factors that trigger the VSS to create a copy instantly. Thus , the archiving process will be launched in the following situations: -Before installing drivers -Before installing Windows components (for example, DirectX) -Before you upgrade the operating system

Turnkey solutions

There are many software solutions from different companies that allow you to backup or archive data by using VSS. Below is a list of the most common ones : -Acronis True Image -Paragon Drive Backup -Leo Backup

Compatibility

While the different NTFS versions have a certain degree of both forward and backward compatibility, there are certain issues when mounting newer NTFS volumes containing persistent shadow copies in older versions of Windows. This affects dual-booting, and external portable hard drives. Specifically, the persistent shadow copies created by Windows Vista on an NTFS volume are deleted when Windows XP or Windows Server 2003 mount that NTFS volume. This happens because the older operating system does not understand the newer format of persistent shadow copies.Likewise, System Restore snapshots created by Windows 8 are deleted if they are exposed by a previous version of Windows.[6]

References

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  1. "Volume Snapshot Service (VSS)". Glossary. Symantec. Retrieved 2 May 2013. 
  2. "Volume Shadow Copy Service Overview". MSDN Library. Microsoft. 5 November 2012. Retrieved 2 May 2013. 
  3. VSS [Electronic resource]: From https://www.symantec.com/: - Access mode: https://www.symantec.com/security_response/glossary/define.jsp?letter=v&word=volume-snapshot-service-vss
  4. VSS [Electronic resource]: From https://msdn.microsoft.com: - Access mode: https://msdn.microsoft.com/en-us/library/windows/desktop/aa384649(v=vs.85).aspx
  5. VSS [Electronic resource]: From https://technet.microsoft.com: - Access mode: https://technet.microsoft.com/en-us/windowsserver/bb405951.aspx
  6. VSS [Electronic resource]: From http://forum.openmediavault.org/: - Access mode: http://forum.openmediavault.org/index.php/Thread/7331-GUIDE-Windows-Previous-Versions-and-Samba-Btrfs-Atomic-COW-Volume-Shadow-Copy/