Path: utzoo!attcan!sobmips!uunet!aplcen!samsung!usc!rutgers!att!tsdiag!pedsga!jeffj
From: jeffj@pedsga.UUCP (Jeff Jonas)
Newsgroups: comp.periphs
Subject: Re: Disks with Multiple, Independent Sets of Heads
Message-ID: <1021@pedsga.UUCP>
Date: 15 Nov 89 18:30:48 GMT
References: <5995@wiley.UUCP>
Reply-To: jeffj@pedsga.UUCP (Jeff Jonas,TF x7560)
Organization: Concurrent Computer Corp., Tinton Falls, N.J.
Lines: 37

As already posted, IBM has a large disk with two sets of heads.
I think the main obstacle is cost.
The emphasis for disks are
	cost
	capacity
	size
	speed
	power consumption
PCs and consumers want low cost and low power consumption.
Portables will trade cost for smaller size.

Multiple heads will greatly reduce the average seek time, but
there's another trick used there too.
Say I have a disk with 1000 cylinders.
With one set of heads, the head must move the entire of the surface of
the disk from edge to edge.
Some disks now have multiple heads per arm.
Just placing a second head in the middle of the arm means that
when the arm is retracted heads are on cylinders 0 and 500,
and fully extended they are on cylinders 499 and 999.
The arm moves only half the surface of the disk.
Logically, the extra set of heads are handled as other
tracks, so 10 surfaces with 2 heads per surface appears to be
20 surfaces with 1 head per surface.
No need for another linear motor and all the related circuitry.

Another consideration is media wear.
I heard that in the never ending quest for greater disk capacity,
the heads are no longer flying or floating on the disk, but
actually riding on a layer of lubricant.
Keeping the number of heads to a minimum reduces wear.

yes, it would be neat, but there are other solutions that are
more cost effective.

Jeffrey Jonas
jeffj@pecnos.tinton.ccur.com