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From: lamaster@pioneer.arpa (Hugh LaMaster)
Newsgroups: comp.arch
Subject: Re: Disk rotational speed vs. striping vs. parallel heads
Message-ID: <2586@ames.arpa>
Date: Tue, 25-Aug-87 18:34:20 EDT
Article-I.D.: ames.2586
Posted: Tue Aug 25 18:34:20 1987
Date-Received: Thu, 27-Aug-87 02:27:33 EDT
References: <2432@ames.arpa> <3721@well.UUCP> <2838@phri.UUCP> <155@dolphy.UUCP> <611@gumby.UUCP> <222@casemo.UUCP>
Sender: usenet@ames.arpa
Reply-To: lamaster@ames.UUCP (Hugh LaMaster)
Organization: NASA Ames Research Center, Moffett Field, Calif.
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In article <222@casemo.UUCP> brian@casemo.UUCP (Brian Cuthie ) writes:

>I didn't realize we were talking about a single threaded machine.  I guess
>it makes less of a difference when one task has to wait for the disks anyway.
>

>
>Sorry again.  I mis-read ths context of the statement.  I spent several
>years designing disk controlers and just assumed we were talking about 
>bus bandwidth.  I oftern forget that most people are actually interesed
>in throughput. Thus two different Mbytes/s.

Big Iron users (Cray, Cyber 205, etc.) need speed.  You can multiprogram
up to a point (4 to 6 tasks in memory per processor, for example), but
beyond that point disk "throughput" doesn't help, because Big Iron jobs
usually contain some very large memory tasks that won't fit in memory with
each other.  The only way to make these faster is speed.  Now, as was pointed
out, there are two kinds of speed:  speed on random accesses, and speed on
sequential accesses.  Fortunately, a lot of big jobs can be structured to
reduce the amount of randomness (and number of seeks).  So, in the big
machine world, the CDC 819 (four parallel heads, 36 Mbit/sec peak speed) was
popular.  The newer drives (CDC, Ibis) run at around 100 Mbit/sec peak speeds.
These drives are sometimes used WITH striping also, either with or without the
help of the operating system.  Because even these speeds are not enough, the
Cray X-MP comes with an solid state "disk" which runs at up to 10Gigabit/sec
rates.  It is also much, much, much faster at random accesses than a magnetic
disk.  An effective aggregate transfer rate of 200MBit/sec is a reasonable
requirement for running at job at 100MFLOPS.  Some newer machines are expected
to run at actual job speeds of 1 GFLOPS.





  Hugh LaMaster, m/s 233-9,  UUCP {seismo,topaz,lll-crg,ucbvax}!
  NASA Ames Research Center                ames!pioneer!lamaster
  Moffett Field, CA 94035    ARPA lamaster@ames-pioneer.arpa
  Phone:  (415)694-6117      ARPA lamaster@pioneer.arc.nasa.gov


                 "IBM will have it soon"


(Disclaimer: "All opinions solely the author's responsibility")