Windows Server 2008: The Definitive Guide

Using Previous Versions

Previous versions (née shadow copies) are a relatively new technology within Windows products that enable a server to take snapshots of documents on a disk to record their states at certain points in time. If a user accidentally deletes or otherwise overwrites a file, she can open a version the server saved earlier in time, thereby eliminating the need to either re-create her work or contact the help desk to get them to restore the file from the most recent backup. When the previous versions feature is enabled on a disk, clients connecting to a share on that disk will be able to view and access previous point-in-time copies of either individual files or entire directories.

Further benefits lurk beneath the surface of this feature, however. The service behind shadow copies, called the Volume Shadow Copy Service (VSS), is actually responsible for a newly developed application programming interface (API) that allows server-based applications such as Exchange, SQL, and backup programs to take advantage of the benefits of shadow copies. Perhaps the most famous example is a backup that skips open files, either because they are currently open by a user or because they are locked by another process. In the past, this resulted in incomplete backups, either because the backup process halted in midstream because of this unrecoverable error, or because the process skipped the open file. If the open file is, say, your Exchange email database, that's not necessarily a good ...