Description:

Unionfs is a stackable unification file system, which can appear to
merge the contents of several directories (branches), while keeping
their physical content separate. Unionfs is useful for unified source
tree management, merged contents of split CD-ROM, merged separate
software package directories, data grids, and more. Unionfs allows any
mix of read-only and read-write branches, as well as insertion and
deletion of branches anywhere in the fan-out.