From owner-freebsd-current  Sat Dec  4 15:11:50 1999
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To: Mike Smith <msmith@freebsd.org>
Subject: Re: Mounting one FS on more than one system 
Cc: match@elen.utah.edu, current@freebsd.org
In-reply-to: Your message of "Sat, 04 Dec 1999 13:53:26 PST."
             <199912042153.NAA04820@mass.cdrom.com> 
Date: Sat, 04 Dec 1999 12:44:43 -0800
From: Kirk McKusick <mckusick@flamingo.McKusick.COM>
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	To: match@elen.utah.edu
	cc: current@freebsd.org, mckusick@mckusick.com
	Subject: Mounting one FS on more than one system
	In-reply-to: Your message of "Sat, 04 Dec 1999 10:10:20 MST."
		     <19991204170659.B8DEE5420@ee.elen.utah.edu> 
	Date: Sat, 04 Dec 1999 13:53:26 -0800
	From: Mike Smith <msmith@freebsd.org>

	(moved to -current where there are more eyes that are interested)

	> P.S. Mike, at comdex I spoke to you some about clustering two 
	> computers and one RAID array, remember? You mentioned that 
	> someone had pursued that avenue some, perhaps not to a working 
	> solution, but I don't remember who. Can you (or anyone else) point 
	> me to the guilty parties? We would like to pick up the work and run 
	> with it for a while.

	Yup, I remember.  I also remember going through my stack
	of business cards wondering whether I remembered to get
	one from you; obviously not.  8)

	The sticking issue that we discussed was allowing more than
	one system to mount a given filesystem; I seemed to recall
	that Kirk has spoken about this before, and there may be
	some folks here (or Kirk himself, also copiedd) who may
	have some more input on the topic.  Once this is resolved,
	everything else is (relatively!) straightforward...

	-- 
	\\ Give a man a fish, and you feed him for a day. \\  Mike Smith
	\\ Tell him he should learn how to fish himself,  \\  msmith@freebsd.org
	\\ and he'll hate you for a lifetime.             \\  msmith@cdrom.com

Mounting on more than one system is generally problematical unless
you are willing to have all systems read-only. The problem is cache
coherence between the machines. If one changes a block, the other
machines will not see it. Basically, this is why we have the NFS
filesystem. That lets a disk be mounted on one machine, but shared
out to others. If you wanted to write a protocol that would allow
for multiple machines, then you would need to have some central
coordinator running some sort of coherency protocol with a complexity
akin to that of NFS.

	Kirk McKusick


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