UnionFS

From Free net encyclopedia

UnionFS is a file system that brings together the files and directories of separate file systems, known as branches, into a single logical file system. The different branches may be read-only or read-write file systems. When mounting the branches the priority of one branch over the other is specified. So when both branches contain a file with the same name one gets priority over the other. It is generally not recommended for production use.

The original UnionFS implementation is in Plan 9 from Bell Labs operating system, which uses union mounts extensively to build custom namespaces per user/processes.

The Linux implementation of UnionFS is sometimes used as an ad-hoc snapshotting system. On LiveCDs it is used as a way of overlaying a writeable file system on top of read-only media. In KNOPPIX for example the union of the file system on the CD-ROM or DVD may be made with the file system contained in an image file called knoppix.img on a USB memory stick which has priority over the read-only filesystem. The user sees then a logical file system where it is possible to add and change files anywhere. What happens on the physical level is that if a file which resides in the read-only branch is replaced by a new version it is put into the image file. As the union file system merges the two branches transparently the user just sees the new version.

UnionFS is also available in many BSD variants, like FreeBSD, NetBSD and DragonFly BSD.

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