RE: Is there a performance advantage of adding a second parity set?

We have 3 parity sets on our 520 (HA Box) an we just lost two 35 Gig Drives in the same parity set..

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of John Jones
Sent: Monday, February 15, 2010 8:42 AM
To: Midrange Systems Technical Discussion
Subject: Re: Is there a performance advantage of adding a second parity set?

If you add 6 drives at once the parity should spread across four of them as you presume.

I don't know that the performance difference would be measurable but I would hazard a guess that 2 parity sets would be better than one. With just one set those 8 parity drives will be spending more and more time performing parity writes for the additional drives; on average they will be busier which could induce delays when the OS wants to read from them. More parity sets spreads the parity writes across more drives sort of like how the i scatter-loads data to begin with. The number of parity writes is unchanged but the busy-ness of the drives that have parity duty is reduced.

Also, a second parity set increases your reliability as you could lose one drive in each set and still be operational.

On modern RAID cards, what is the maximum number of drives per parity set anyway? I thought it topped out at 15 or 18.

On Mon, Feb 15, 2010 at 8:18 AM, <rob@xxxxxxxxx> wrote:

We have a system with 17 drives. We are going to add six 70GB drives.
If I add them two at a time they will add them to the existing parity
set. If I add them all at once it will start a new parity set.
Starting a new parity set effectively ties up one of the 70GB drives
to raid striping. We really don't need the space though - we just have
70GB drives laying around. Without the new drives we are only at 50%
used. The existing parity set has the striping across 8 drives. I
take it the new set would have the striping across 4 drives?

These six 4327's would go into a 5787. Into P1-D1 through P1-D6.
P1-D7 through P1-D12 are 4328's. P2-D1 through P2-D5 and P2-D7
through
P2-D11 are 4328's. Leaving P2-D6 and P2-D12 open.

Rob Berendt
--
Group Dekko Services, LLC
Dept 01.073
Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com

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