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| UnionFS is a Linux filesystem service which emulates a union mount for Linux file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system. Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem. The different branches may be read-only or read-write file systems, so that writes to the virtual, merged copy can be directed to a specific real filesystem. This leads to one other useful purpose for union mounts, which is to make a filesystem appear as writeable, but without actually allowing writes to change the filesystem, also known as copy-on-write. This may be desirable when the media is physically read-only, such as in the case of CDROMs, or for using a single common template for a number of filesystems, or perhaps for security reasons. The Linux implementation, UnionFS, is sometimes used as an ad-hoc snapshotting system. When mounting 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. UnionFS in Linux is generally not recommended for production use. 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. |