Path: utzoo!attcan!uunet!husc6!mailrus!purdue!decwrl!pyramid!prls!mips!cprice
From: cprice@mips.COM (Charlie Price)
Newsgroups: comp.arch
Subject: Re: Why no disks with two HDAs ?
Keywords: diskdrives, headdiskassemblies
Message-ID: <2447@winchester.mips.COM>
Date: 21 Jun 88 04:31:22 GMT
References: <2351@uklirb.UUCP> <2440@winchester.mips.COM> <8dly9ed7S71010boqkE@amdahl.uts.amdahl.com>
Reply-To: cprice@winchester.UUCP (Charlie Price)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 54

In article <8dly9ed7S71010boqkE@amdahl.uts.amdahl.com> mat@amdahl.uts.amdahl.com (Mike Taylor) writes:
>In article <2440@winchester.mips.COM>, cprice@mips.COM (Charlie Price) writes:
>> 
>> Two follow-on big disks were the 3370 and 3380.
>> In addition to having two HDAs, they took the step of having
>> two separate actuators (head arms) inside each HDA.
>> For some reason, IBM still wanted to build physically large HDAs
>> with a large number of platters and a lot of data so they made
>> the math come out right by having two actuators.
>> The current technology really favors building the same sort
>> of system out of a larger number of smaller HDAs.  It isn't
>> clear to me that IBM's choice made technological sense.
>
>They probably had a manufacturing technology for the 14" platters
>that they didn't want to replace.  I expect their sums said that
>using old platters and developing new head technology - the thin-film
>head - was the best economic choice.
>-- 
>Mike Taylor                        ...!{ihnp4,hplabs,amdcad,sun}!amdahl!mat
>

A good thought, and perhaps that is part of it.
It turns out that they made a questionable call if that was the basis
for the decision.

I believe the 3370 uses the old industry standard 14" platters
(it has been a while) but it is a lower density product than the 3380.
Supposedly the original work on the 3380 suggested that the standard
platters might not by stable enough to get the desired track density.
The disks are 1) not perfectly flat in the first place and 2) deform some
in a real HDA when they are clamped into a fixed stack at the inner diameter,
heated up, and spun at 3600 RPM.  You can tolerate a certain amount of
"surface terrain" on a disk, but as your track density goes up the 
head flying height perforce gets lower and the sensitivity to surface
variations increases.
IBM decided to go ahead with standard platters because changing to a new
kind of platter would cost a lot of money in tooling and such -- like $20Meg.
After a while, they decided they had to spend the $20Meg and thereby
slipped the delivery of these disks (like 15 months or more).
The platters they use are quite thick -- like 1/4 inch.

Perhaps they wanted to take the conservative route and this ended up
being more expensive than they expected.  On the other hand, these
new platters are probably a fair bit better for reliable high capacity
drives.  IBM certainly didn't lose money on 3380.

StorageTek was quite worried about this substrate change for a while
because they just didn't have the resources to create a whole new substrate
(STC mostly buys disks from other people, like Dysan.)
In the end, STC was able to use the standard disks and do reasonably well.
HDA reliability related to disk stability has never been a problem in
their 8380 HDAs.

Charlie Price      cprice@mips.com    {decwrl,ames}!mips!cprice