Union Mounts in 4.4BSD-Lite

This paper describes the design and rationale behind union mounts, a new filesystem-namespace management tool available in 4.4BSD-Lite. Unlike a traditional mount that hides the contents of the directory on which it is placed, a union mount presents a view of a merger of the two directories. Although only the filesystem at the top of the union stack can be modified, the union filesystem gives the appearance of allowing anything to be deleted or modified. Files in the lower layer may be deleted with whiteout in the top layer. Files to be modified are automatically copied to the top layer.

This new functionality makes possible several new applications including the ability to apply patches to a CD-ROM and eliminate symbolic links generated by an automounter. Also possible is the provision of per user views of the filesystem, allowing private views of a shared work area, or local builds from a centrally shared read-only source tree.