Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ptsfa!ihnp4!inuxc!iuvax!bsu-cs!dhesi
From: dhesi@bsu-cs.UUCP
Newsgroups: comp.arch
Subject: Re: Disk Striping (description and references) plus class brief
Message-ID: <981@bsu-cs.UUCP>
Date: Fri, 14-Aug-87 17:51:25 EDT
Article-I.D.: bsu-cs.981
Posted: Fri Aug 14 17:51:25 1987
Date-Received: Sun, 16-Aug-87 08:41:55 EDT
References: <2432@ames.arpa> <3721@well.UUCP> <2838@phri.UUCP> <505@mtuxo.UUCP>
Reply-To: dhesi@bsu-cs.UUCP (Rahul Dhesi)
Organization: CS Dept, Ball St U, Muncie, Indiana
Lines: 41
Summary: Centrifugal force is probably the limiting factor

In article <505@mtuxo.UUCP> jlw@mtuxo.UUCP (J.WOOD) writes:
[about the issue of running disk drives at much higher speeds]

>Disks cannot be put in a vacuum using current thinking since
>the heads literally fly along the surface of the disk.
>. . . .The head tries to fly into the disk.  Then when it gets
>microscopically close ground effects take over and keep the head
>from actually crashing into the disk surface (most of the time :-) ).

[and operating a disk drive in a vacuum would prevent the use of
airflow between the head and the disk surface from maintaining a
tight tolerance.]

I think the main limiting factor keeping disk drive speeds low is
the tremendous centrifugal/centripetal force that acts at high speeds.
I've used ultracentrifuges that run at 60,000 rpm and above, and the
rotors have to be *very* carefully machined.  Even small scratches can
lead to spots of strain that will eventually cause the rotor to break
up.  I've also seen what the inside of an ultracentrifuge looks like
after a rotor breaks up -- you don't want to be nearby when that
happens, just in case the half-inch steel casing doesn't hold up
(though it's designed to).

Ultracentrifuge rotors are kept small.  All the ones I've seen that
were used at high speed were about 8 to 10 inches in diameter.  I
suspect anything much larger would be awfully difficult to spin fast
without breaking up.

Even if you could make the disk platter strong enough, I wonder what it
would do to the magnetic coating to be subjected to a lot of Gs.  Might
it not want to flow a little towards the edge of the platter?

Finally, some speculation.  We know that "information", which we
normally think of being a purely abstract quantity, is actually closely
related to entropy and therefore to heat, which we can actually feel.
I wonder if "information" has mass too?  Just a little?  Enough that
bits of information would tend to move towards the outer tracks?  This
may sound like a crackpot idea, but don't forget, there was a time when
they would have called you crazy if you had said that light had mass.
-- 
Rahul Dhesi         UUCP:  {ihnp4,seismo}!{iuvax,pur-ee}!bsu-cs!dhesi