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From: dwc@homxc.UUCP (D.CHEN)
Newsgroups: comp.arch
Subject: Re: Disk Striping (description and references) plus class brief
Message-ID: <898@homxc.UUCP>
Date: Fri, 14-Aug-87 16:10:19 EDT
Article-I.D.: homxc.898
Posted: Fri Aug 14 16:10:19 1987
Date-Received: Sun, 16-Aug-87 05:41:24 EDT
References: <2432@ames.arpa> <3721@well.UUCP>
Organization: AT&T Bell Laboratories, Holmdel
Lines: 33

In article <3721@well.UUCP>, rchrd@well.UUCP writes:
> 
> 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,

can't you seal the disk in a vacuum?  isn't that what happens in
winchesters?  i don't know, just asking.

> 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 was doing a quick analysis of disk stripping and made a small
observation:  with multiple disks, the average rotational latency
approches that of an entire rotation instead of 1/2 or 1/3 rotation
(i forget the numbers).

danny chen
homxc!dwc