Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!zaphod.mps.ohio-state.edu!ncar!ico!rcd
From: rcd@ico.isc.com (Dick Dunn)
Newsgroups: comp.arch
Subject: disk rotation speed
Summary: various interacting factors (which are changing "even as we speak")
Message-ID: <1990Aug1.220440.20727@ico.isc.com>
Date: 1 Aug 90 22:04:40 GMT
References: <2635@mindlink.UUCP> <10048@pt.cs.cmu.edu> <1990Jul31.200043.5189@nlm.nih.gov>
Organization: Interactive Systems Corporation, Boulder, CO
Lines: 31

usenet@nlm.nih.gov (not a useful address) writes:

> While on the subject, why do disks spin so slowly?  At 3600 RPM, you
> have an intrinsic delay of up to 16 msec waiting for the platter to
> come around, no matter how fast the heads move.  You could increase the
> speed of a 5 1/4" disk by an order of magnitude without worrying about
> g forces...

There has been some erratic motion toward higher rotation rates, and it's
become more of a trend in the last year or so.  I think 5400 will be the
next pseudo-standard.

One of the difficulties is that if you speed up rotation, you either lose
density or increase data rate.  The bulk of the effort on disk drives in
the last decade has gone to increasing capacity, particularly in smaller
drives where there's a constraining external form factor.  What's really
happened is that the ability to increase linear density (useful FCPI) has
grown at (*very* roughly) the same pace as the ability to handle increasing
data rate...thus the rotation speed has stayed constant.  Increasing data
rate is a problem for two reasons:  reliability in the read/write circuitry
(the heads and the analog stuff out there close to them), and conformance
to standards for the drive interface.  There's not much sense in busting
your * to get, say 4 Mb/s if you have to shove it through a 3 Mb/s hose to
the CPU.  Data rate increases have happened, but too slowly to allow for
much change in rotation speed.  In fact, you can see some high-end drives
designed with dual actuators to reduce rotational latency...the tradeoff
here is that it was apparently cheaper to add a second set of heads, servo,
r/w logic and all, than to deal with the problems of faster rotation.
-- 
Dick Dunn     rcd@ico.isc.com  -or-  ico!rcd          (303)449-2870
   ...Software, not suits.