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From: rchrd@well.UUCP (Richard Friedman)
Newsgroups: comp.arch
Subject: Re: Disk Striping (description and references) plus class brief
Message-ID: <3721@well.UUCP>
Date: Mon, 10-Aug-87 13:24:40 EDT
Article-I.D.: well.3721
Posted: Mon Aug 10 13:24:40 1987
Date-Received: Tue, 11-Aug-87 05:55:35 EDT
References: <2432@ames.arpa>
Reply-To: rchrd@well.UUCP (Richard Friedman)
Organization: Whole Earth 'Lectronic Link, Sausalito, CA
Lines: 27


One more comment about disk striping:  there is a real limiting
factor to disk technology, and it is not the speed of light (the
limiting factor to CPU technology) but rather the speed of sound.
If you try to make a disk go too fast in an attempt to improve
transfer rates, you approach Mach 1 in the turbulent flow around
the surface of the disk, and the resulting shock wave destroyes
the disk, literally.   By spreading the data across many platters,
you achieve a kind of parallelism in I/O.  I have written some
software packages using asynch I/O on the Cray that attempt this
sort of thing and it is very successful for large blocks of data.
But there is always a trade-off.  Imagine a situation where a
transfer of a single record of a few million 8-byte words is
broken down into a simultaineous transfer of a number of 
partitions of this recordthis can be transferred asynchronously
so that each partition is running inparallel on its own disk.
Now you have improved the transfer rate, but you have also
increased the overall I/O "interference" and it may slow down
due to increased system activity!   There's no free lun}inch,
it seems.
}i
  
Richard Friedman    Pacific-Sierra Rsearch (Berkeley)
-- 
...Richard Friedman [rchrd]                         
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