To help counter the disk space-stealing effects of storing multiple Remote Installation Services (RIS) images, Microsoft included Single Instance Storage (SIS) in Windows 2000. The file sets that make up various images on a RIS server are almost identical, so physically storing the files within these sets in duplicate across the different image folders is wasteful. SIS locates duplicate files stored within RIS images and replaces them with pointers to a master copy of the file. Thus, only the new and unique files contained within a RIS image consume disk space.
Although SIS and RIS are independent services that don't require each other's presence to operate, Win2K automatically installs SIS at the same time you install RIS and its ancillary services. SIS consists of two components: the SIS Filter driver and the SIS Groveler service. When the SIS Groveler service discovers a duplicate instance of a file that is 32KB or larger, SIS moves a copy of the file to the \SIS Common Store folder on the volume and gives the file a name that consists of the file's globally unique ID (GUID) and has a .sis extension. SIS then replaces the original file with an NTFS reparse point that points to the copy of the original file. When a process attempts to access the original file, the reparse point directs the process to the instance of the file in the \SIS Common Store folder.
Microsoft designed SIS to be CPU-friendly and run during off-peak hours. The SIS Groveler service runs at a slower rate during the hours immediately following its installation so that it can spend this time profiling the system performance to determine how much CPU time it can use without adversely affecting system performance. Thus, don't be surprised if, at first, the space utilization on your RIS image volume is more than you expected. The Groveler service will eventually catch up and consolidate duplicate files. You can accelerate this process by forcing the Groveler service to run in a more CPU-intensive mode of operation. To do so, use the Expand command to expand the grovctrl.ex_ file from the Win2K Server CD-ROM's \i386 folder as grovctrl.exe and place it in a folder on your server's hard disk (e.g., \%systemroot%\system32). After you expand the file, use the following command line to launch it:
Grovctrl f
This command forces the service to run in foreground mode until it completes its initial operation, after which the service automatically returns to CPU-friendly mode.