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From: rcd@ico.UUCP (Dick Dunn)
Newsgroups: comp.arch
Subject: mach my day
Message-ID: <1430@ico.UUCP>
Date: Fri, 21-Aug-87 02:13:35 EDT
Article-I.D.: ico.1430
Posted: Fri Aug 21 02:13:35 1987
Date-Received: Sat, 22-Aug-87 18:13:11 EDT
Organization: Interactive Systems Corp, Boulder, CO
Lines: 18
Keywords: subsonic

This stuff about speed of sound vs disk speed seems to be getting out of
hand.  Reality-check time.

The largest disk platter in common use is 14 inches in diameter.  The
most common max rotational speed is 3600 rpm, which is 60 rps.  This gives
a maximum (at OD) head:disk velocity of
	14 * pi * 60 / 12	feet/sec
which is about 220 feet/sec, or about 0.2 of the speed of sound.

So would someone explain why we're worrying about hypersonic disk heads?
If we went to 10000 rpm, we'd still be just over Mach 0.5.

Another way to look at it:  If we can realize a bit density of, say 800 bpi
(the effective bit density of 6250 bpi, 9-track tape) on a disk, the
effective data rate at mach 1 is somewhat over 10 Mbit/sec.
-- 
Dick Dunn    {hao,nbires,cbosgd}!ico!rcd  (NOT CSNET!)   (303)449-2870
   ...Never offend with style when you can offend with substance.