The SIS Common Store and Common-Store Files

All backing files maintained by single-instance store (SIS) backup are called common-store files. One common store exists on each SIS-maintained volume and contains all of the common-store files that exist on that volume. This directory is located in the root directory of the volume and is named "\SIS Common Store". The common store is implemented as a directory that is protected against normal user access, because common-store files are intended to be accessed directly only by SIS backup API functions and not backup and restore applications.

The properties of a common-store file are the following:

• A common-store file may have one or more links pointing to it.
• After it is created, the contents of a common-store file never change.
• The names of common-store files are globally unique—that is, they are unique across all volumes across all systems in the world, and the binding between a common-store file name and its data is globally static.

When SIS is enabled on a volume, one copy of the duplicate files is then created to the common store, and all of the duplicate files are replaced with SIS links pointing to the common-store file. For more information, see Enabling or Disabling SIS on a Volume in the TechNet documentation.

When one of the SIS links is accessed for a write operation, the SIS filter first restores the original contents of the SIS link file from the common-store file, the reparse point linking the SIS link file to the common store is removed, and then the write operation is performed on the original destination file.

The common-store file is removed only when the last SIS link pointing to it is deleted.