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. They originated with Plan 9 and its concept of union directories.
- 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.
- Wright, Charles P.; Jay Dave, Puja Gupta, Harikesavan Krishnan, Erez Zadok, and 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.
- Blunck, Jan (May 2009). "VFS based Union Mount (V3)". lwn.net. Retrieved 21 December 2009.
- About GlusterFS. November 2009. Retrieved 4 March 2013.
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