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.
Famous quotes containing the words union and/or mount:
“Let’s have the Union restored as it was, if we can; but if we can’t, I’m in favor of the Union as it wasn’t.”
—Artemus Ward (1834–1867)
“As every pool reflects the image of the sun, so every thought and thing restores us an image and creature of the supreme Good. The universe is perforated by a million channels for his activity. All things mount and mount.”
—Ralph Waldo Emerson (1803–1882)