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-[[!meta copyright="Copyright © 2009 Free Software Foundation, Inc."]]
-
-[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
-id="license" text="Permission is granted to copy, distribute and/or modify this
-document under the terms of the GNU Free Documentation License, Version 1.2 or
-any later version published by the Free Software Foundation; with no Invariant
-Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
-is included in the section entitled
-[[GNU Free Documentation License|/fdl]]."]]"""]]
-
-[[!meta title="Union Mounts"]]
-
-When setting a translator on Hurd -- similar to mounting a file system on UNIX
--- the new node(s) exported by the translator are obscuring the original node
-where the translator is set, and any nodes below it in the directory tree. The
-translator itself can access the underlying node (which is a very nice feature,
-as it allows translators presenting the contents of the node in a different
-format); but it's no longer accessible from the "outside".
-
-Plan9 has a feature where a file system can be mounted in union mode: the new
-file system doesn't obscure the mount point in this case, but instead the
-contents are combined. (This feature has also been under discussion in Linux
-for a couple of years now, under the label "VFS-based union mounts".)
-
-This kind of union mounts is generally useful, as it's sometimes more
-convenient than unioning existing filesystem locations with unionfs -- it's not
-necessary to mount a file system that is to be unioned at some external
-location first: just union-mount it directly at the target location.
-
-But union mounts also allow creating passive translator hierarchies: If there
-is a passive translator on a parent node, and further passive translators on
-child nodes, the union mount allows the child nodes with the further translator
-settings still to be visible after the parent translator has started.
-
-This could be useful for device nodes for example: let's say we have an
-ethernet multiplexer at /dev/veth. Now the virtual subnodes could all be
-directly under /dev, i.e. /dev/veth0, /dev/veth1 etc., and explicitely refer to
-the main /dev/veth node in the translator command line. It would be more
-elegant however to store the virtual nodes direcly below the main multiplexer
-node -- /dev/veth/0, /dev/veth/1 etc.
-
-There are two possible approaches how union mounts could be implemented in the
-Hurd. The first one is to let the various translators handle union mounts
-internally, i.e. let them present the underlying nodes to the clients in
-addition to the actual nodes they export themselfs. This probably can be
-implemented as some kind of extension to the existing netfs and diskfs
-libraries.
-
-The other possible apporach is less efficient and probably more tricky, but
-probably also more generic: create a special unionmount translator, which
-serves as a kind of proxy: setting the union-mounted translator on some
-internal node; and at the actual mount location, presenting a union of the
-nodes exported by this translator, and the nodes from the underlying file
-system.
-
-The goal of this project is implementing union mounts using either of the
-approaches described above. (Though it might be useful initially to prototype
-both for comparision.) The ethernet multiplexer shall serve as an example use
-case -- any changes necessary to allow using it with the union mount
-functionality are also to be considered part of the task.