RE: What is a disk arm?

I remember "head per track" disks where nothing swung at all!! No seek time, just rotational delay!

There've been attempts, I think, to minimize rotational delay by two arms swinging over the same track but rotational delay is usually much shorter than seek time in any case, so it didn't improve much. I do remember disks that had an arm swinging from the outside in and another from the inside out. In that case, each arm was serving its own set of tracks. That was a long time ago, before density got so high you could store so much on a 3.5 inch disk. Seek time is pretty quick when you only have to swing across a 1.75 inch path (on average, half of that). The multi-armed ones I remember were something like 12 or 14 inch disks. Remember the Winchester drive anybody?

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Roberto José Etcheverry Romero
Sent: Thursday, June 13, 2013 4:11 PM
To: Midrange Systems Technical Discussion
Subject: Re: What is a disk arm?

Charles, A disk may have had multiple heads (and in fact, most nowadays still have more than 1 platter) but i've never heard of a disk with more than one ARM per se with arm being the entire assembly. I wondered if they could add another arm to the same disk (would have to change the disk size) to get 2 arms on one enclosure, but i suppose it wasnt deemed necessary...


On Thu, Jun 13, 2013 at 5:54 PM, Charles Wilt <charles.wilt@xxxxxxxxx>wrote:

As I understand it....

Back before my time..disks had multiple independent arms..thus "disk arms"
vs. "disks"

now-a-days, 1 disk = 1 disk arm

Charles


On Thu, Jun 13, 2013 at 4:24 PM, Buck Calabro <kc2hiz@xxxxxxxxx> wrote:

It's often said that there's no stupid question, but I think I'm
about to test this adage. I've got my cable set between the UPS and
the 720 (8202-E4C). I got the CPI0962 (UPS now attached) message.
I don't have a UPS power handling program written yet, so I want to
set QUPSDLYTIM to a value that will allow the machine to write main
store to disk before crashing. I look at

http://pic.dhe.ibm.com/infocenter/iseries/v7r1m0/index.jsp?topic=%2Frz
ahr%2Frzahrdlytimco.htm

(Determining the value of QUPSDLYTIM) and I can see that the value
is heavily dependent on the number of disk arms and main storage.
WRKSYSSTS and a calculator tell me I have the 32GB we paid for :-)
WRKDSKSTS tells me I have 8 198C disk units and 12 19B0 disk units.
If that's 20 arms, I'm hosed - the table says it'll take (worst
case) 58 minutes to write 32GB RAM to disk (!) [1]

And so, I don't think those 20 disk units equal 20 disk arms but my
Google-fu has failed me in finding a conversion table. I tried
Power 720, 8202-E4C, 198C, 19B0. Is 'disk arm' an imaginary unit of
measure like CPW?
--buck

[1] These numbers seem crazy even if worst case. It's really going
to take an hour to write 32GB of data from RAM to disk?
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