Single Instance Storage

Applies To: Windows Storage Server 2008

The Single Instance Storage (SIS) feature reduces the amount of space that is used to store data on a volume. SIS does this by replacing duplicate files with logical links that point to a single copy of the file in the SIS Common Store, which is a hidden folder that is located in the root directory of the volume.

SIS consists of two primary components that together maintain a database of file signatures. These components include:

  • Groveler service - The Groveler service scans the hard-disk volumes on a server for duplicate copies of files. If the service locates duplicate copies of files, the information about the duplicates is sent to the Single Instance Storage Filter. The Groveler service runs as a user-level service.

  • Single Instance Storage Filter - The Single Instance Storage Filter is a file system filter service that manages duplicate copies of files on hard-disk volumes. When notified by the Groveler service of duplicate copies of files, this component copies one instance of a duplicate file into a central folder. The duplicate is then replaced by a link (a reparse point) to the central copy. The link file contains information about the original file, such as its current location, size, and attributes. The Single Instance Storage Filter runs in kernel mode.

The Single Instance Storage Filter service cannot be stopped. If this service is disabled, the linked files are not accessible. If the central folder is deleted, the linked files can become permanently inaccessible. If you stop the Groveler service, the files cannot be automatically linked, but the existing linked files can still be accessible.

You can enable SIS on a maximum of 128 volumes per computer. SIS cannot act upon any files that are referenced through junction points, and it cannot be used with any file system except the NTFS file system. SIS will not process files that are 32 kilobytes or less in size.

If you need to access data that is stored on a SIS volume, which might be required for backup and recovery operations, you must either run or have installed Single Instance Storage Filter on your computer.

Backup and recovery by using SIS has the following requirements:

  • The backup software used must support SIS-enabled volumes.

  • The SIS volume, SIS Common Store folder, and reparse points (links) to the files must be restored to a Windows 2000 NTFS version 5.0 (or later) file system or partition that supports reparse points or junction points.

  • The Single Instance Storage Filter must be installed or enabled to access the data in the SIS volume.

  • The backup program must be capable and configured to backup and restore the reparse points or junction points (links) to the files, and the SIS volume and the SIS Common Store folder must be selected.


Windows Server Backup can be used to backup SIS-enabled volumes. In this case, it is recommended to do a volume-level backup as file-level and folder-level backups are not optimized by SIS.

For more information about SIS, see Single-Instance Store and SIS Backup.

In this section