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

When I first saw your question I had to ask myself why you would go thru all this work. When I worked for an AS400 maintenance company I use to have some free time to play around on different systems. I did a performance test and found no performance benefit.

If it were me I would not waste my time.


Jim


--- On Fri, 8/22/08, Charles Wilt <charles.wilt@xxxxxxxxx> wrote:

From: Charles Wilt <charles.wilt@xxxxxxxxx>
Subject: Re: Performance Impact of going from 2 parity drives to 4 parity drives in a 4-disk RAID set
To: "Midrange Systems Technical Discussion" <midrange-l@xxxxxxxxxxxx>
Date: Friday, August 22, 2008, 2:35 AM
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.

Charles

On Thu, Aug 21, 2008 at 3:17 PM, Raul A. Jager W.
<raul@xxxxxxxxxx> wrote:

Are you changing from RAID 1 to RAID 5? RAID 5 usualy

requires at least

3 drives to be usefull.

Charles Wilt wrote:

All,

related to my post about starting/stopping RAID....

The reason for stopping/starting RAID is to spread

out the parity

information from the current 2 drives to all 4

drives in a 4-disk RAID

set.

However, one of the team members on the system

admin team mentioned

that he was told, by IBM, there would be no

noticable performance

impact of moving from 2 parity drive to 4 in a

4-disk RAID set.

Both of us find that surprising.

Does anybody have any first hand expirence to

confim or deny this claim...

Thanks,

Charles Wilt

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