Previous Versions and Shadow Copies

Windows 7 can also restore earlier versions of files so that users can quickly recover a file that has been accidentally modified, corrupted, or deleted. Depending on the type of file or folder, users can open, save to a different location, or restore a previous version. The sections that follow describe the Volume Shadow Copy technology and the Previous Versions user interface.

How Volume Shadow Copy Works

To provide backups for files that are in use, Windows 7 uses the Volume Shadow Copy service, which was first introduced with Windows XP. Volume Shadow Copy mitigates file access between applications and the backup process. In other words, if a backup tool needs to access a file currently in use, Volume Shadow Copy creates a shadow copy of that file and then provides the backup process access to the shadow copy. Figure below illustrates the relationship between Volume Shadow Copy features.

Volume Shadow Copy works with any application. Some applications, however, can communicate directly with the Volume Shadow Copy service to ensure that backed-up files are consistent. If an application keeps several files in use at the same time, they might become inconsistent if two files must be synchronized and one of those files is updated after another is backed up in an earlier state.

Note Application developers can build Volume Shadow Copy integration into their applications using the Volume Shadow Copy Service software development kit (SDK). For more information, go to
http://www.microsoft.com/downloads/details.aspx?FamilyID=0b4f56e4-0ccc-4626-826a-ed2c4c95c871.

To provide backup access to a file that is open and being updated, Volume Shadow Copy needs to be able to make two versions of the file accessible: one that is currently in use by the application and a second that is a snapshot of the file when backup first requested access to a volume shadow copy. Volume Shadow Copy handles this transparently by storing copies of changed files in Volume Shadow Copy storage. Volume Shadow Copy stores a copy of the original state of any modified portion of a file, which allows the original file to be updated without interrupting the backup process. In other words, if a user modifies a file after the backup starts, the file will be in the state it was in when the backup began.