Union Mounts in 4.4BSD-Lite

                         Jan-Simon Pendry
                             Sequent UK

                     Marshall Kirk McKusick
                      Author and Consultant


       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  any-
       thing  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.

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