Single-instance storage

Single-instance storage (SIS) 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 software, data backup and other storage-related computer software.

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 servers 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 backup software, 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 single instance storage, only one copy of a file is written to the backup media therefore reducing space. This becomes more important when the storage is offsite and on cloud storage such as Amazon S3. In such cases, it has been reported that deduplication can help reduce the costs of storage, costs of bandwidth and backup windows by up to 10:1.

Novell GroupWise was built on single-instance storage which accounts for its large capacity.

ISO CD/DVD image files can be optimized to use SIS to reduce the size of a CD/DVD compilation (if there are enough duplicated files) to make it fit into smaller media.

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.

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