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From: kent@xanth.UUCP (Kent Paul Dolan)
Newsgroups: comp.arch
Subject: Re: Disk Striping (description and references) plus class brief
Message-ID: <2091@xanth.UUCP>
Date: Thu, 13-Aug-87 11:26:12 EDT
Article-I.D.: xanth.2091
Posted: Thu Aug 13 11:26:12 1987
Date-Received: Sat, 15-Aug-87 10:40:36 EDT
References: <2432@ames.arpa> <3721@well.UUCP> <2838@phri.UUCP>
Reply-To: kent@xanth.UUCP (Kent Paul Dolan)
Organization: Old Dominion University, Norfolk Va.
Lines: 38
Summary: vacuum inappropriate for disk drives (maybe)

In article <2838@phri.UUCP> roy@phri.UUCP (Roy Smith) writes:
>In article <3721@well.UUCP> rchrd@well.UUCP (Richard Friedman) writes:
>> 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.
>
>	So, why not put the disk in a vacuum?  We've got ultracentrifuges[...]

>	Ultracentrafuges run at about 50 microns of vacuum; for a 36,000
>RPM disk you wouldn't need anywhere near that; a few PSI would probably be
>fine.[...]
>Roy Smith, {allegra,cmcl2,philabs}!phri!roy

Hmmm.  From the little I remember of disk technology, back when the
hardware side of things was still interesting to me, disk heads depend
on the air cushion for spacing from the platter surface.  The heads
are described as "flying" on a surface effect air cushion.  Without
that aerodynamic effect, head positioning would require much more
accurate mechanisms, and disk platters would have much smaller
manufacturing tolerances and more stringent materials requirements to
avoid original or age related "waves" in the surface.

Still, there ought to be some gain from drawing a partial vacuum, in
that the energy transfer from heads to surface would be less if less
air were being plowed out of the way above Mach 1, so that at some
level of vacuum, perhaps the remaining (supersonic) air would fly the
heads, but not destroy them or the platters.  Of course, there is
still the problem of (standing ?) waves between the platters and the
case...

Usual answer - more research required.  ;-)



Kent, the man from xanth.