Path: utzoo!yunexus!geac!syntron!jtsv16!uunet!seismo!sundc!pitstop!sun!amdcad!ames!ncar!tank!uxc!uxc.cso.uiuc.edu!urbsdc!aglew
From: aglew@urbsdc.Urbana.Gould.COM
Newsgroups: comp.arch
Subject: Re: The NeXT Problem [Disk Swapping
Message-ID: <28200215@urbsdc>
Date: 17 Oct 88 13:45:00 GMT
Article-I.D.: urbsdc.28200215
References: <10293@eddie.MIT.EDU>
Lines: 40
Nf-ID: #R:eddie.MIT.EDU:10293:urbsdc:28200215:000:1835
Nf-From: urbsdc.Urbana.Gould.COM!aglew    Oct 17 08:45:00 1988


..> Multiple heads per arm
>>[Henry Spencer]
>>the Fujitsu Eagle has two
>>heads per surface on its linear actuator.  Not a new idea.
>
>(Darn, I wish I had the balls to say "Uh, Henry,"...)
>
>Actually, mulitiple heads per surface is an old idea.  It used to be quite
>common to have "head-per-track" drives.  Of course these were generally
>single platter machines.  I believe the reason for shifting to a multiple
>platter "head-per-surface" scheme was cost.
>
>Actually, there are still a few applications where head-per-track drives
>are still used.  I believe Univac (err, Sperry, errr, Unisys) makes one
>for military aircraft.  100 MB as I remember.
>
>The problem with placing multiple heads on a *moving* actuator is that each
>head adds mass.  It may not have to move as far, but it's slower getting
>started (for a given servo system) and slower stopping.  Any physicists
>want to talk about the tradeoffs between moving a large mass a short distance,
>and moving a small mass a long distance?
>
>Rick Farris            rfarris@serene.cts.com     voice         (619) 259-6793

I suppose that head per track drives have gotten scarce because of 
smaller track spacing?

I've always wondered why disk arms are not simply lined all the way along
with read/write heads at the closest spacing optical lithography can 
provide. Either in silicon (maybe a segment every 4 inches) or some other
substrate. True, you couldn't make such a long arm fly evenly, but 
you'd just have to lower one part at a time, which would probably be easier
than moving the whole arm. Or, fly the heads high, read from multiple heads
simultaneously, and remove cross track interference from the signal for the 
head you want to read.

NB. I nothing *NOTHING* about disk drives. The above is idle speculation.

aglew@gould.com (now Motorola MCD CUDC)