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RE: RAID 6 rebuild time - can I slash it?



fixed

Larry,

I had a performance issue with drives in the CEC, when all drives did NOT contain stripping, net result was the drives with stripping were "hot drives"
I end parity and started parity with striping across all drives, it did make a huge difference.

With everything I read about Raid6 performance, not good, especially in the CEC, that could be disastrous from a performance point of view.

Also, most drives failures I've had occurred gradually, system called home, alerted you a drive is about to fail. A hot spare in this case will not add any benefit.
In this case, you must force the failure, replace the drive.

Paul

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of DrFranken
Sent: Thursday, March 20, 2014 9:27 AM
To: Midrange Systems Technical Discussion
Subject: Re: RAID 6 rebuild time - can I slash it?

Rob,

You are correct but be careful. If an 'event' occurred such as lightning for example that took out three drives, your RAID 6 statement would be wrong. Problem is the hot spare didn't have a chance to become part of the protection since all three went at one time. Although there is a 3 in 8 chance one of the drives that got clobbered WAS the hot spare so you still win. I don't like those odds though. :-)

If I read Sue right (rare I suppose) the need for more arms weighs more heavily than the need for spares in the RAID-6 space especially.

Rule:
No Protection:
Read=1 operation Write=1 operation
Mirroring:
Read=1 operation (2 candidates) Write=2 operations
RAID 5:
Read=1 operation Write=2 Reads and 2 Writes (4 ops)
RAID 6:
Read=1 operation Write=3 Reads and 3 Writes (6 ops)

Reducing the number of arms with hot spare in an 8 drive set takes away 12.5% of the IOPS of the array and parks 'em. But RAID 6 added 50% more ops to a write.

With RAID 5 and Hot spare you're exposed during a failure but write performance is better do to the lower ops.

With RAID 6 and NO Hot spare you ARE Protected during a single failure and you get those IOPS back helping to compensate for the extra ops needed to support RAID 6.

Have you ever measured the performance difference between 4 drives striped and 8 drives striped? In my experience it matters very little when all drives are healthy and seems only to impact performance while a drive is down. Of course hot spare minimizes the duration of that time.

- Larry "DrFranken" Bolhuis

www.frankeni.com
www.iDevCloud.com
www.iInTheCloud.com

On 3/20/2014 8:33 AM, rob@xxxxxxxxx wrote:
Sue,

Raid 6 would mean you'd have to lose at least two drives to be wiped out.
Raid 6 with hot spare means you'd have to lose at least three drives,
right?

Kind of like, which is better:
Raid 5 with a hot spare, or
Raid 6 with no hot spare
Either way, you'd have to lose two drives to be fried. I would think
that Raid 6 would be better if there was a risk the second drive would
be lost while the hot spare for raid 5 was becoming active and still rebuilding.

With Raid 5 you lose the space of one drive to striping (spread out
across all drives). How much do you lose to Raid 6?

Is there a performance degradation from removing one of your disk
drives of a 8 drive SCSI raid set to become hot spare?
I would think there would be two performance hits. One, dropping from
8 drives supporting the raid stripe down to only 4. The other
performance hit would be one less disk arm assisting while it's just
sitting there waiting to be hot spare.


Rob Berendt

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