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From: jans@mako.UUCP (Jan Steinman)
Newsgroups: net.arch
Subject: Re: I don't believe your statements about multiprocessors
Message-ID: <761@mako.UUCP>
Date: Mon, 13-May-85 13:09:08 EDT
Article-I.D.: mako.761
Posted: Mon May 13 13:09:08 1985
Date-Received: Wed, 15-May-85 00:20:06 EDT
References: <7202@Glacier.ARPA>
Reply-To: jans@mako.UUCP (Jan Steinman)
Distribution: net
Organization: Tektronix, Wilsonville OR
Lines: 22
Summary: 

In article <7202@Glacier.ARPA> reid@Glacier.ARPA (Brian Reid) writes:
>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.  Writes are performed in
parellel, but the task of reading is given to whichever disk is currently
positioned closest to the desired data, effectively cutting average read 
access in half.

Some of Tandem's recent upstart competition have tried to spread this
philosophy to other peripherals, but is was determined that the difficulty of
reading the alternating-line listings produced by parallel printers offset the
speed advantage. (I can't believe I wrote that! :-)

-- 
:::::: Jan Steinman		Box 1000, MS 61-161	(w)503/685-2843 ::::::
:::::: tektronix!tekecs!jans	Wilsonville, OR 97070	(h)503/657-7703 ::::::