Re: Performance Impact of going from 2 parity drives to 4 paritydrives in a 4-disk RAID set

["Raul A. Jager W." <raul@xxxxxxxxxx>]

How do you control which drives have the parity?  I had the idea that 
the data and the parity are spread over all the disks in the RAID set.

Charles Wilt wrote:

> Chris,
>
> To to be sure...
>
> You are saying there's a performance improvement going from a 4-disk
> RAID 5 that only uses 2 parity drives to a 4-disk RAID 5 that uses all
> 4 drives for parity.
>
> That's what I expected, but IBM is telling us otherwise.
>
> We've already made the move from 3-disk RAID 5 to 4-Disk RAID 5.
>
>
> Thanks!
> Charles
>
>
> On Thu, Aug 21, 2008 at 4:27 PM, Chris Bipes
> <chris.bipes@xxxxxxxxxxxxxxx> wrote:
>  
>
> > Yes your performance will increase significantly.   WRKDSKSTS and see
> > what the % busy is for the drives.  If it is way out of balance, then
> > you will notice the change.  If not, don't bother.
> >
> >
> > Chris Bipes
> > Director of Information Services
> > CrossCheck, Inc.
> >
> >
> > -----Original Message-----
> > From: midrange-l-bounces@xxxxxxxxxxxx
> > [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Charles Wilt
> > Sent: Thursday, August 21, 2008 1:06 PM
> > To: Midrange Systems Technical Discussion
> > Subject: Re: Performance Impact of going from 2 parity drives to 4
> > paritydrives in a 4-disk RAID set
> >
> > No.
> >
> > It's already a RAID 5, it will continue to be RAID 5.
> >
> > Originally, it was a 3-disk RAID 5.  Thus parity info was only on two
> > disks.
> >
> > We added a 4th disk, but the OS does not automatically change the number
> > of disks used for parity data.
> >
> > If you were to create a 4 disk RAID 5 array from scratch, parity would
> > be spread across all 4 disks.
> >
> > So the question is is it worth the trouble of going into DST to stop and
> > restart RAID 5 in order to get the OS to spread the parity across all 4
> > disks instead of just 2 disks.
> > --
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