Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!unisoft!gethen!farren
From: farren@gethen.UUCP (Michael J. Farren)
Newsgroups: comp.arch
Subject: Re: Disk drives -- speed of?
Message-ID: <887@gethen.UUCP>
Date: 22 Apr 88 05:57:49 GMT
References: <2746@sundc.UUCP> <76700012@uiucdcsp> <2759@sundc.UUCP>
Reply-To: farren@gethen.UUCP (Michael J. Farren)
Organization: There's Unix there in Oakland
Lines: 31

In article <2759@sundc.UUCP> bwong@sundc.UUCP (Brian Wong) writes:
>
>OK, here's the $64k question: why is it that a mechanical speed is being
>bottled up by an electrical speed?  

It's not, really.  The data rate coming off of the disk is a function
of two things: the rate of speed of rotation of the disk, and the
density of the bit packing on the media.  You could have the same data
rates on a disk turning at 1 RPM, if you could pack the bits in there
tighter.

There's a couple of things operating here.  First, most disk controllers
are limited in bandwidth.  This has to do, as much as anything, with
the corresponding limitations in the I/O channel they are working with.
Second, recording technology is also limited in bandwidth - this has
to do with the fact that it takes time to cause a flux change in a
magnetic medium.  While this can be improved, there are limits there.
I don't see that the 3600 RPM speed is cast in stone, but it does
seem to fit quite well with the current state of the art in magnetic
recording and data processing (less well with the latter).

Personally, I'd always assumed that the disks rotated at 3600 RPM
because it was easier to design to that speed - 3600 RPM equates to
60 RP/second, which happens to be the line frequency most common in
countries which manufacture disk drives (read: USA).  Don't know if
this is just computer folk etymology or not, though.

-- 
Michael J. Farren             | "INVESTIGATE your point of view, don't just 
{ucbvax, uunet, hoptoad}!     | dogmatize it!  Reflect on it and re-evaluate
        unisoft!gethen!farren | it.  You may want to change your mind someday."
gethen!farren@lll-winken.llnl.gov ----- Tom Reingold, from alt.flame