Re: Disk issue: More arms versus improved hardware

[Larry Bolhuis <lbolhuis@xxxxxxxxxx>]

Jeff,

  You got it *half* right. In fact some vendors were shipping fast 8G
drives as 4G drives and 17ss as 8s. They were just using half the
cylinders thus limiting the maximum seek distance and (somewhat)
improving the average seek time.

  Assuming the full capacity of the drive is used (as it normally is)
then with twice as much 'stuff' on the disk you'll need to access it
twice as often on average, so there goes your advantage. With an
architecture like OS/400 that automatically spreads data across storage
this is true in practice as well.  SO the drive/interface/controller
combination needs to net out to be twice as fast to perform the same as
two drives of half the capicity.

  Remember: "Many arms make light work."

  - Larry

jeff_carey@baxter.com wrote:
> 
> I too am wondering about this.  If you have more than twice the data on
> the same sized disk, it would seem that the seek time for an arm to find
> the data would be about half (at least).  Wouldn't that help mitigate the
> effect of having fewer arms?  Also, each arm is a point of failure as
> well, so would the performance increase of more arms outweigh the
> decreased time between failures?  I haven't done the math yet, but it's
> probably worth considering.
> 
> Jeff Carey
> Technical Specialist

-- 
Larry Bolhuis           | Cogito Ergo Vendo iSeries 
Arbor Solutions, Inc.   |
(616) 451-2500          |               (I think, therefore I buy iSeries.)
(616) 451-2571 -fax     |
lbolhuis@arbsol.com     |        #3  1951-2001
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