Unionfs: A Stackable Unification File
System

This project builds 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.

或另一个类似的选项aufs.