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