Path: utzoo!attcan!uunet!husc6!mailrus!ames!oliveb!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: <2440@winchester.mips.COM>
Date: 20 Jun 88 07:14:56 GMT
References: <2351@uklirb.UUCP>
Reply-To: cprice@winchester.UUCP (Charlie Price)
Organization: MIPS Computer Systems, Sunnyvale, CA
Lines: 72


> R. Kirchner
> 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.

A head disk assembly, or HDA, is the part that *must* be replaced
when you have a head crash.  It is an assembly that contains
a spindle with the disks mounted on it, the magnetic read/write heads
mounted on a movable actuator (called a head-arm)
and (at least part of) a servo positioning system to move the head arm.

Several IBM "disk drives" are boxes with more than one HDA in them.
The earliest I'm aware of was the 3350.  This used regular old HDAs.

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.
These two arms don't share any tracks,
they each access half of the disk surfaces on the HDA.
[Having two independent heads over the same track isn't cost effective.]
These actuators are positioned independently and there are independent
read/write paths to each head arm.  You can read or write on the
separate head arms independently.  The servo is fairly fast
and the data transfer rate is 3 MBytes/sec.

The IBM HDA is *huge*.  The head arms are located on opposite sides
of the vertically-mounted HDA (the disks are vertical).
It reminds me, for some reason, of a Volkswagen engine.

> - why doesn't anybody else produce drives with two HDAs ?
  > ( or does somebody whom I do not know ?, and then why ? )
> . Is it not worth the effort, patented by IBM, or what is the reason ?

Several IBM plug-compatible manufacturers produce drives that
are functionally compatible with the IBM drives.
I worked for Storage Technology Corp (also StorageTek)
for several years so I know the most about their drives.
The StorageTek versions of the HDA use the same basic geometry,
but their actuators are located side-by-side on one side
of the disk platters.
Other manufacturers do similar things.
I believe the Amdahl drives, manufactured by Fujitsu, use 4 separate
HDAs (something like the Super Eagle HDA).

Why go to all this trouble?
I don't know why IBM chose this approach, but one serious problem that
they wanted to address was the problem of having too much data being
serviced by a single head arm.  Much of what IBM mainframes do is
"data intensive" and these systems need speedy access to data.
Disks are terribly slow compared to the processors; particularly
physical positioning time.
To have fast IO you want many disks and you want to keep them
all busy.  The larger the amount of data under a given head arm,
the more likely that you will get into a situation where the
system bottleneck is access to that data.
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 original 3380 had 1.25 Mbyte HDAs, the 3380E has 2.5 MByte HDAs,
and the current drives (number unknown) are 3.75 Mbyte HDAs.

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.
STC built the HDA with two actuators because at the time you
couldn't sell a plug-compatible product to an IBM customer
if it wasn't very similar to the IBM offering.


Charlie Price   cprice@mips.com    {decwrl|ames}!mips!cprice
MIPS Computer Systems / 930 Arques /  Sunnyvale, CA 94087 / (408) 720-1700