Union mount: Difference between revisions
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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.<ref name="fsl"/><ref name="lwn1"/><ref name="lwn2"/><ref name="lwn3"/> |
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.<ref name="fsl"/><ref name="lwn1"/><ref name="lwn2"/><ref name="lwn3"/> |
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Generally one of the filesystems will be mounted read-write, while other filesystems are mounted read-only. |
Generally one of the filesystems will be mounted read-write, while other filesystems are mounted read-only. |
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Union mounts are implemented by union filesystems such as [[UnionFS]] and [[AUFS]], frequently used by [[Live CD]]s. They originated with [[Plan 9 from Bell Labs#Union directories|Plan 9]] and its concept of ''union directories''.<ref name="lwn4"/> |
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Union mounts were invented in ca. 1990, appearing in [[Berkeley Software Distribution#4.4BSD-Lite|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 Microsystems|Sun]], respectively.<ref name="usenix"/> They are also a central concept in [[Plan 9 from Bell Labs|Plan 9]], which replaces several Unix staples with union mounts (e.g., several directories unioned together at a single <code>/bin</code> directory replace the <code>PATH</code> variable).<ref name= "use-of-namespaces">{{cite web | url= http://doc.cat-v.org/plan_9/4th_edition/papers/names | title = The Use of Name Spaces in Plan 9 | first1= R. |last1= Pike |authorlink= Ken Thompson |first2= D. | last2 = Presotto | first3 = K. |last3=Thompson |first4=H. |last4= Trickey |first5= P. |last5= Winterbottom |work= Bell Labs | publisher = [http://cat-v.org/ cat-v.org] |accessdate= 2011-12-05}}</ref> |
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If one wants to mount different filesystems distributed among a network (rather than being located on the same machine), [[GlusterFS]] offers means of doing so.<ref name="glusterfs"/> |
If one wants to mount different filesystems distributed among a network (rather than being located on the same machine), [[GlusterFS]] offers means of doing so.<ref name="glusterfs"/> |
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<ref name="lwn2">{{Cite journal|title=Union file systems: Implementations, part I|journal=lwn.net|date=March 2009|first=Valerie|last=Aurora|coauthors=Henson|url=http://lwn.net/Articles/325369/|accessdate=21 December 2009}}</ref> |
<ref name="lwn2">{{Cite journal|title=Union file systems: Implementations, part I|journal=lwn.net|date=March 2009|first=Valerie|last=Aurora|coauthors=Henson|url=http://lwn.net/Articles/325369/|accessdate=21 December 2009}}</ref> |
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<ref name="lwn3">{{Cite journal|title=Unioning file systems: Implementations, part 2|journal=lwn.net|date=April 2009|first=Valerie|last=Aurora|coauthors=Henson|url=http://lwn.net/Articles/327738/|accessdate=21 December 2009}}</ref> |
<ref name="lwn3">{{Cite journal|title=Unioning file systems: Implementations, part 2|journal=lwn.net|date=April 2009|first=Valerie|last=Aurora|coauthors=Henson|url=http://lwn.net/Articles/327738/|accessdate=21 December 2009}}</ref> |
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<ref name="lwn4">{{Cite journal|title=VFS based Union Mount (V3)|journal=lwn.net|date=May 2009|first=Jan|last=Blunck|url=http://lwn.net/Articles/333589/|accessdate=21 December 2009}}</ref> |
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<ref name="glusterfs">{{Cite journal|title=About GlusterFS|date=November 2009|url=http://www.gluster.org/about/|accessdate=4 March 2013}}</ref> |
<ref name="glusterfs">{{Cite journal|title=About GlusterFS|date=November 2009|url=http://www.gluster.org/about/|accessdate=4 March 2013}}</ref> |
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}} |
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Revision as of 19:14, 11 September 2013
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]
If one wants to mount different filesystems distributed among a network (rather than being located on the same machine), GlusterFS offers means of doing so.[7]
References
- ^ a b Pendry, Jan-Simon (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.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Wright, Charles P. "Versatility and Unix Semantics in a Fan-Out Unification File System". Stony Brook University Technical Report FSL-04-01b. Retrieved 25 November 2007.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Aurora, Valerie (March 2009). "Unioning file systems: Architecture, features, and design choices". lwn.net. Retrieved 21 December 2009.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Aurora, Valerie (March 2009). "Union file systems: Implementations, part I". lwn.net. Retrieved 21 December 2009.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Aurora, Valerie (April 2009). "Unioning file systems: Implementations, part 2". lwn.net. Retrieved 21 December 2009.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Pike, R.; Presotto, D.; Thompson, K.; Trickey, H.; Winterbottom, P. "The Use of Name Spaces in Plan 9". Bell Labs. cat-v.org. Retrieved 5 December 2011.
{{cite web}}: External link in|publisher= - ^ "About GlusterFS". November 2009. Retrieved 4 March 2013.
{{cite journal}}: Cite journal requires|journal=(help)
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