Union mount

From Wikipedia, the free encyclopedia
Jump to: navigation, search

A union mount is a mount that allows several filesystems to be mounted at one time, appearing to be one filesystem.[1]

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.[2][3][4][5]

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.[1] 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 PATH variable).[6]

Similarly, GlusterFS offers a possibility to mount different filesystems distributed across a network, rather than being located on the same machine.[7]


  1. ^ a b Pendry, Jan-Simon; Marshall Kirk McKusick (December 1995). "Union Mounts in 4.4BSD-Lite". Proceedings of the USENIX Technical Conference on UNIX and Advanced Computing Systems: 25–33. Retrieved 25 November 2007. 
  2. ^ 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. 
  3. ^ Aurora, Valerie; Henson (March 2009). "Unioning file systems: Architecture, features, and design choices". lwn.net. Retrieved 21 December 2009. 
  4. ^ Aurora, Valerie; Henson (March 2009). "Union file systems: Implementations, part I". lwn.net. Retrieved 21 December 2009. 
  5. ^ Aurora, Valerie; Henson (April 2009). "Unioning file systems: Implementations, part 2". lwn.net. Retrieved 21 December 2009. 
  6. ^ 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. 
  7. ^ "About GlusterFS". November 2009. Retrieved 4 March 2013.