Re: What is a disk arm?

[DrFranken <midrange@xxxxxxxxxxxx>]

Correct. The 9332 family of drives - models 400 and 600 were supported 
on AS/400 - had two sets of heads each covering 1/2 of the platters (or 
possibly 1/2 of the tracks on all the platters, can't be certain any 
longer. :-)  ).   As a consequence the system saw 2 arms per drive of 
1/2 the capacity or 200/200 and 300/300 MB, yes MB.  When Mirroring and 
Checksum (precursor to RAID) came you had to be very careful how you 
picked your arms or you could have multiple components in the same drive 
and thus no protection!

        - Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 6/13/2013 4:54 PM, Charles Wilt 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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