Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!agate!saturn!raspail!bga@shamash.cdc.com
From: raspail!bga@shamash.cdc.com (Bruce Albrecht)
Newsgroups: comp.os.research
Subject: Re: Question for the week -- distributed Multics?
Message-ID: <6238@saturn.ucsc.edu>
Date: 5 Feb 89 05:11:05 GMT
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Approved: comp-os-research@jupiter.ucsc.edu


CDC's NOS/VE is a Multics derivative, with a memory mapped file system with
shared segments, and segments may be shared by multiple machines.  The only
catch is that currently the segment may be shared for read access by all
machines, or have exclusive access by a single machine for write access.
Theoretically, the shared segment could be available for concurrent write
access (or non-exclusive access for write) on more than one machine, but it
would be a real performance killer.

CDC's NOS/VE has a file server, which is a process that communicates with 
the file servers on the other machines.  When a shared segment is accessed, 
the file server sends requests for reads to the machine on which the segment 
actually resides (i.e., the machine to which the disk is connected.)  If 
concurrent write accesses were allowed, the file servers would have to be 
responsible for invalidating pages on other mainframes, just as cache is 
invalidated on a multiple CPU machine.  As long as there is some mechanism 
provided by the file servers to emulate it, even test_and_set instructions 
could be done across mainframes.

I think the multi-mainframe file server for NOS/VE has been available for
a year or so.  We don't use it for all of our development systems, though,
because our development systems tend to be running slightly different variants,
and they only run it when two mainframes are scheduled to be in sync, OS-wise.
In a few weeks, though, our communications development organization will be
migrating to 2 dual CPU 860's, running with the multi-mainframe file server.
I don't believe they support load-leveling between mainframes, but I think
it's a natural extension of the multi-mainframe shared segments.