Single-instance storage

See also:

Single instance storage is a system's ability to keep one copy of content that multiple users or computers share. It is a means to eliminate data duplication and to increase efficiency, SIS is frequently implemented in file systems, e-mail server products, data backup and other storage-related solutions.

In the case of an e-mail server, single-instance storage would mean that a single copy of a message is held within its database whilst individual mailboxes access the content through a reference pointer. However there is a common misconception that the primary benefit of single instance storage in mail server solutions is a reduction in disk space requirements. The truth is that its primary benefit is to greatly enhance delivery efficiency of messages sent to large distribution lists. In a mail server scenario disk space savings from single instance storage are transient and drop off very quickly over time.

When used in conjunction with a backup solution, single instance storage can reduce the quantity of archive media required since it avoids storing duplicate copies of the same file. Often identical files are installed on multiple computers, for example operating system files. With solutions that use single instance storage, only one copy of a file is written to the backup media therefore reducing space.

Novell GroupWise was built on single-instance storage which accounts for the large data stores that GroupWise is able to achieve.

The NTFS file system supports single-instance storage. The file-based Windows Imaging Format introduced in Windows Vista also supports single-instance storage. Single-instance storage has been a feature of Microsoft Exchange Server since version 4.0 and is also present in Microsoft's Windows Home Server. It is protected by several patent applications, including United States Patent numbers 6389433 and 6477544.

Microsoft [states], "SIS works by searching a hard disk volume to identify duplicate files. When SIS finds identical files, it saves one copy of the file to a central repository, called the SIS Common Store, and replaces other copies with pointers to the stored versions." Windows Storage Server 2003 R2 has SIS capabilities but is limited to OEM OS system installs.

SIS is related to system wide file duplication search and multiple file instance detection tools such as the P2P application Bearshare (5.n Versions and below) but differs in that SIS reduces storage utilization automatically and creates and retains symbolic linkages, whereas Bearshare allows for manual deletion of duplicates and associated user level file system, Windows Explorer type of icon links.