weblog: Added Mail from olaf about the HUrd and plan9.

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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="Major differences between Plan9 and the Hurd"]]
+
+There are some similarities between the Hurd and Plan 9 regarding the file 
+system handling -- but there are also very fundamental differences which go 
+far beyond monolithic vs. microkernel design:
+
+- The Hurd is UNIX (POSIX) compatible
+
+- While (almost) all services are attached to the file system tree, not
+  all services actually export a file system interface!  
+  
+  Personally, I advocate using FS-based interfaces as much as possible.
+  Yet, there are some cases where they get very arward and/or
+  inefficient, and domain-specific interfaces simply make a lot more
+  sense.  
+  
+  Also, some Hurd services are indeed used to implement the file systems
+  in the first place -- they work below the FS level, and obviously
+  can't use an FS interface!
+
+- File systems are completely decentralized -- clients always talk to
+  the FS servers directly, without any central VFS layer. (I don't think
+  that's the case in Plan 9?)  
+  
+  This offers much more flexibility -- the way the FS interfaces
+  themselfs work can be modified. Many things can be implemented as
+  normal translators, that would require special VFS support on other
+  systems. (Extended attributes, VFS-based union mounts, local
+  namespaces, firmlink, magic file name suffixes etc.)
+
+- The system design allows users and applications to change almost all
+  aspects of the system functionality in the local environment easily
+  and without affecting other parts of the system.  
+  
+  (This is possible with Plan 9 to some extent; but the Hurd allows it
+  at a much lower level -- including stuff like the filesystem
+  interfaces, access control mechanisms, program execution and process
+  management, and so on.)
+
+I hope I didn't forget any major differences...
+
+*(copied from a Mail in the 
+[[l4-hurd|mailing_lists#l4-hurd]] 
+list).*