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.
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?
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.
Read=1 operation Write=1 operation
Read=1 operation (2 candidates) Write=2 operations
Read=1 operation Write=2 Reads and 2 Writes (4 ops)
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
On 3/20/2014 8:33 AM, rob@xxxxxxxxx wrote:
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,
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.
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