Re: What is a disk arm?

[Roberto José Etcheverry Romero <yggdrasil.raiker@xxxxxxxxx>]

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=%2Frzahr%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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