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

[DrFranken <midrange@xxxxxxxxxxxx>]

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