The SIS Common Store and Common-Store Files (Part 3)

What you’ve been waiting for—part 3 of our 4-part SIS series:

All backing files maintained by SIS 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 called SIS Common Store. The common store is implemented as a directory that is restricted to allow access only to the system. Only the SIS filter and backup applications need access to this directory. The common-store file is removed only when the last SIS link pointing to it is deleted.

The SIS Groveler
The SIS groveler searches for and combines files that are identical on the NTFS volume. It then reports those files to the SIS filter driver. It does most of its work when the system is not busy. This is referred to as “background mode.” It is possible to run the groveler at maximum capacity by using the Sisadmin.exe tool. This is referred to as “foreground mode.” After the groveler completes its work in foreground mode, it resumes normal operation in background mode.

Some notes about the SIS Groveler:

  • The groveler does not dynamically recognize newly configured SIS volumes. The Sisadmin.exe tool will restart the groveler appropriately as configuration changes are made.
  • The JET database that the groveler uses limits the groveler to managing six volumes at the same time.
  • The groveler will only attach to NTFS volumes that are mounted as a drive letter. There is no support for SIS linking volumes mounted using mount points.

Scanning process
The SIS groveler uses the update sequence number (USN) Journal to know when files on a volume have been created or changed. If the journal wraps (for example, the groveler misses some journal entries), the groveler will automatically re-scan the entire volume and recreate the signature database.