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

Raul A. Jager W. wrote:

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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Depends on how many drives were used to start teh raid5 set.
Starting From DST on leaving the defaults
3 drives = 2 with parity and data and 1 with data only
4, 8 or 10 Drives all have parity and data
5,6,7 = 4 with parity and data 1,2 or 3 with data Only
On later releases there is an option that allows you semi control the above specifing Performance, Capacity or Balanced ( the default )