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	microkernel/mach/external_pager_mechanism.mdwn

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+[[!meta copyright="Copyright © 2008, 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="Allow Using unionfs Early at Boot"]]
+
+In [[UNIX]] systems, traditionally most software is installed in a common directory
+hierarchy, where files from various packages live beside each other, grouped by
+function: user-invokable executables in `/bin`, system-wide configuration files
+in `/etc`, architecture specific static files in `/lib`, variable data in
+`/var`, and so on.  To allow clean installation, deinstallation, and upgrade of
+software packages, GNU/Linux distributions usually come with a package manager,
+which keeps track of all files upon installation/removal in some kind of
+central database.
+
+An alternative approach is the one implemented by GNU Stow: each package is
+actually installed in a private directory tree.  The actual standard directory
+structure is then created by collecting the individual files from all the
+packages, and presenting them in the common `/bin`, `/lib`, etc. locations.
+
+While the normal Stow package (for traditional UNIX systems) uses symlinks to
+the actual files, updated on installation/deinstallation events, the Hurd
+[[hurd/translator]] mechanism allows a much more elegant solution:
+[[hurd/translator/stowfs]] (which is actually a special mode of
+[[hurd/translator/unionfs]]) creates virtual directories on the fly, composed
+of all the files from the individual package directories.
+
+The problem with this approach is that unionfs presently can be launched only
+once the system is booted up, meaning the virtual directories are not available
+at boot time.  But the boot process itself already needs access to files from
+various packages.  So to make this design actually usable, it is necessary to
+come up with a way to launch unionfs very early at boot time, along with the
+root filesystem.
+
+Completing this task will require gaining a very good understanding of the Hurd
+boot process and other parts of the design.  It requires some design skills
+also to come up with a working mechanism.
+
+Possible mentors: Carl Fredrik Hammar (cfhammar)