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USENIX Technical Program - Abstract - WinsSys - August 2000
Single Instance Storage in Windows 2000
William J. Bolosky, Scott Corbin, David Goebel, and John R. Douceur, Microsoft Research
Abstract
Certain applications, such as Windows 2000’s Remote Install
service, can result in a set of files in which many different files have the
same content. Using a traditional file system to store these files separately
results in excessive use of disk and main memory file cache space. Using hard or symbolic links would eliminate
the excess resource requirements, but changes the semantics of having separate
files, in that updates to one “copy” of a file would be visible to users of
another “copy.” We describe the Single
Instance Store (SIS), a component within Windows® 2000 that
implements links with the semantics of copies for files stored on a Windows
2000 NTFS volume. SIS uses
copy-on-close to implement the copy semantics of its links. SIS is structured as a file system filter
driver that implements links and a user level service that detects duplicate
files and reports them to the filter for conversion into links. Because SIS links are semantically identical
to separate files, SIS creates them automatically when it detects files with
duplicate contents. This paper
describes the design and implementation of SIS in detail, briefly presents
measurements of a remote install server showing a 58% disk space savings by
using SIS, and discusses other possible uses of SIS.
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