Rather than mounting each filesystem at a different place in the directory hierarchy, a union mount overlays the filesystems, creating a unified hierarchy. Thus, any given directory (or "folder") in the resulting filesystem may contain files and subdirectories from any or all of the underlying filesystems.
Generally one of the filesystems will be mounted read-write, while other filesystems are mounted read-only. Union mounts are implemented by union filesystems, such as UnionFS and AUFS, frequently used by Live CDs.
Union mounts were invented in ca. 1990, appearing in 4.4BSD-Lite; the authors of the BSD implementation cite previous work on the 3-d filesystem and the Translucent File System (TFS) done at Bell Labs and Sun, respectively. They are also a central concept in Plan 9, which replaces several Unix staples with union mounts (e.g., several directories unioned together at a single
/bin directory replace the
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- Wright, Charles P.; Jay Dave; Puja Gupta; Harikesavan Krishnan; Erez Zadok; Mohammad Nayyer Zubair. "Versatility and Unix Semantics in a Fan-Out Unification File System". Stony Brook University Technical Report FSL-04-01b. Retrieved 25 November 2007.
- Aurora, Valerie; Henson (March 2009). "Unioning file systems: Architecture, features, and design choices". lwn.net. Retrieved 21 December 2009.
- Aurora, Valerie; Henson (March 2009). "Union file systems: Implementations, part I". lwn.net. Retrieved 21 December 2009.
- Aurora, Valerie; Henson (April 2009). "Unioning file systems: Implementations, part 2". lwn.net. Retrieved 21 December 2009.
- Pike, R.; Presotto, D.; Thompson, K.; Trickey, H.; Winterbottom, P. "The Use of Name Spaces in Plan 9". Bell Labs. cat-v.org. Retrieved 2011-12-05.
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