Path: utzoo!utgpu!tmsoft!spectrix!clewis
From: clewis@spectrix.UUCP (Chris Lewis (It's loose again!))
Newsgroups: comp.arch
Subject: Re: Why no disks with two HDAs ?
Keywords: diskdrives, headdiskassemblies
Message-ID: <672@spectrix.UUCP>
Date: 21 Jun 88 15:41:45 GMT
References: <2351@uklirb.UUCP>
Reply-To: clewis@spectrix.UUCP (Chris Lewis (It's loose again!))
Organization: Spectrix Microsystems Inc., Toronto, Ontario, Canada
Lines: 29

In article <2351@uklirb.UUCP> kirchner@uklirb.UUCP (Reinhard Kirchner) writes:

>Hello,
>recently I once again learned about IBM disk drives which have two
>head-mechanisms ( Head-Disk-Assemblies , HDA ) per spindle.
>They have several drives of this kind, including the big 3380.

>Now:
>- why doesn't anybody else produce drives with two HDAs ?
>  ( or does somebody whom I do not know ?, and then why ? )

I believe that the CDC Hydra (good name!) has four HDA's, but I could
be mistaken.  On a 27Mbyte/second channels no less.  Yipes!

>. Is it not worth the effort, patented by IBM, or what is the reason ?

The main reasons are that each additional set of heads, head
electronics and data pathways is *very* expensive, and that you don't gain 
anything unless your controller and/or drivers are very intelligent, and 
usually only under pretty high load.

Personally, I'd normally prefer to have twice the capacity on two separate
but overlapping seek drives than occasionally getting up to twice the 
thruput on a single disk.  Mind you, a Hydra would make a nice swapping
or paging disk - almost as good as a head-per-track disk or a drum.
-- 
Chris Lewis, Spectrix Microsystems Inc, Phone: (416)-474-1955
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