To: Kirk McKusick <[EMAIL PROTECTED]>
cc: Mike Smith <[EMAIL PROTECTED]>, [EMAIL PROTECTED],
[EMAIL PROTECTED]
Subject: Re: Mounting one FS on more than one system
In-reply-to: Your message of "Sat, 04 Dec 1999 12:44:43 PST."
<[EMAIL PROTECTED]>
Date: Sun, 05 Dec 1999 09:44:32 +1000
From: George Michaelson <[EMAIL PROTECTED]>
Let me start by saying that I do not have the definitive answers
to all your questions. I'll take a crack at some of them.
Allowing for cache writeback delays, is the speed of
direct-to-shared-disk fast enough that using NFS as an
"abstraction" layer would be faster than any network extant?
The gains come from being able to read data directly from the
disk rather than transferring it across the network. However,
the cost of maintaining cache coherency would be at least as
difficult and bandwidth consuming as a distributed filesystem.
Would it be as fast? would the effort to make this work
exceed the cost of making real networks exist?
As the network speed approaches the disk speed, the gains
would diminish.
It would seem that there might be opportunities to do 'cut
through' in the coding for known-private files after open
(ok, inode allocation/extension has problems) to optimize
them to at-worst 'disk+bits' instead of NFS costs.
The problem is in identifying when private goes to shared. Also
as you point out, new block and inode allocations have to be
centrally coordinated.
If one party mounts -r the FS (eg news spool) then does
this reduce the complexity? eg /usr mounted read-mostly
for a bunch of tightly coupled boxes.
If any machine can write, then all the other machines have to
have some way of keeping their caches consistent with the
machine that did the modification.
If some other protocol is used for interlock, does this
make mmap shares across clusters faster?
Mmap sharing across machines is going to be slow. I have never
been a fan of distributed shared memory as a programming model,
and this does not look like a way of making it run any faster.
-George
--
George Michaelson | DSTC Pty Ltd
Email: [EMAIL PROTECTED] | University of Qld 4072
Phone: +61 7 3365 4310 | Australia
Fax: +61 7 3365 4311 | http://www.dstc.edu.au
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