Path: utzoo!attcan!uunet!snorkelwacker!husc6!bu-cs!encore!pinocchio
From: terryk@pinocchio (Terence Kelleher)
Newsgroups: comp.periphs
Subject: Re: Disks with Multiple, Independent Sets of Heads
Message-ID: <10384@encore.Encore.COM>
Date: 17 Nov 89 13:31:43 GMT
References: <5995@wiley.UUCP> <1021@pedsga.UUCP>
Sender: news@Encore.COM
Reply-To: terryk@pinocchio (Terence Kelleher)
Organization: Encore Computer Corp
Lines: 38
In-reply-to: jeffj@pedsga.UUCP (Jeff Jonas)

>In article <1021@pedsga.UUCP>, jeffj@pedsga (Jeff Jonas) writes:
>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.

This also increases the mass that needs to be moved for seeking and
complicates the mechanical makeup of the arms.  

The NEC 9 inch drives are built this way, but I don't recall that they
seek any faster than other drives with single heads per arm.

>
>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. 
>

I don't recall any drives that put the heads directly on the
lubricant.  I would think this would create a severe heat problem even
if the lubricant could protect the media for an extended priod of
time.  

>
>Jeffrey Jonas
>jeffj@pecnos.tinton.ccur.com
-- 
Terry Kelleher, Encore Computer
Phone: 607-798-9602
Internet: terryk@encore.encore.com