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From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: net.arch
Subject: Re: I don't believe your statements about multiprocessors
Message-ID: <5597@utzoo.UUCP>
Date: Wed, 15-May-85 16:04:53 EDT
Article-I.D.: utzoo.5597
Posted: Wed May 15 16:04:53 1985
Date-Received: Wed, 15-May-85 16:04:53 EDT
References: <7202@Glacier.ARPA>, <761@mako.UUCP>
Organization: U of Toronto Zoology
Lines: 21

> >For loosely-coupled architectures there are sometimes arguments about
> >reliability through redundancy, though they tend not to hold water in
> >practice because of peripherals.
> 
> There are companies who are making piles of money doing exactly this.  Most
> notable is Tandem.  (I'm a former employee.)  The peripherals in a Tandem
> (and I assume, their recent competitors) follow the scheme, with performance
> benefits in the case of disks.  Except for some magic concerning defect
> mapping, the mirrored disks have identical images...

Yes, but most multiprocessor systems do *not* duplicate all the peripherals.
Replication of peripherals is unusual except on systems (like Tandem's)
whose major goal in life is high reliability.  I think Brian's point was
that "reliability through redundancy" is not a significant advantage for
multiprocessors unless peripherals are duplicated too, which they usually
aren't.  I know that C.mmp -- one of the multiprocessor systems that Brian
has worked on -- was plagued by fast-but-unreliable disks and insufficient
funding for full replication.
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,linus,decvax}!utzoo!henry