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Path: utzoo!watmath!clyde!rutgers!ames!ucbcad!ucbvax!sdcsvax!dan
From: dan@sdcsvax.UUCP
Newsgroups: mod.os
Subject: Re: kernel servers vs. user servers.
Message-ID: <2483@sdcsvax.UCSD.EDU>
Date: Sun, 18-Jan-87 22:43:13 EST
Article-I.D.: sdcsvax.2483
Posted: Sun Jan 18 22:43:13 1987
Date-Received: Mon, 19-Jan-87 06:26:43 EST
Sender: darrell@sdcsvax.UCSD.EDU
Organization: Prairie Computing, Madison, Wisconsin
Lines: 38
Approved: mod-os@sdcsvax.uucp

--

>I guess the main reason I designed my distributed file
>system in the kernel was to ensure that existing binaries
>did not have to be recompiled to take advantage of remote
>access.  I don't think this is possible with user-level
>servers, is it?  If it is, could you please direct me to
>technical information about how this is/was/could be done?
>I would be very interested.

   There is a nice little distributed, message passing operating system for
the IBM PC and AT, called QNX.  It has a small kernel for message services
and the clock interrupt, and relies on administrator tasks for everything
else, including file service.  There are various hacks in the default
administrators to optimize things like program loading, but the apparent
behaviour is as described.

   There are various other user-written file system administrators available, 
including one for MS-DOS file systems.  In order to avoid message forwarding
through the default f.s. admin, the library routines that communicate with
it maintain, for each file, the task ID of the administrator handling that
file.  The user administrator, on startup, sends an `adopt disk' message to
the default admin, telling it which disk name (it can be an arbitrary hex
digit between 1 and F) it wants to serve.  When a task first tries to open a 
file on that disk, the default administrator sends the library routine what
amounts to a redirect message, saying, "From now on communicate with task
so-and-so for this file".

   The redirection takes place completely under the covers, so any compiled
program can be used with any file system, as long as it provides services
adequate to that program's needs.


-- 
    Dan Frank
    uucp: ... uwvax!prairie!dan
    arpa: dan%caseus@spool.wisc.edu
--