[ale] Confusing RAID Performance

[Greg Freemyer]

On Wed, Feb 2, 2011 at 6:11 PM, David Tomaschik
<david at systemoverlord.com> wrote:
> On 02/02/2011 04:23 PM, Jeff Hubbs wrote:
>> On 2/2/11 2:15 PM, scott wrote:
>>> Remember that RAID6 is slower than RAID5.  RAID5 calculates the parity
>>> once.  RAID6 does it twice.  This is to make sure that you have parity
>>> protection incase you drop a drive. I would only recommend RAID6 on
>>> large drives (1TB or larger).
>> I sure wouldn't.  For >=~1TB drives, the probability of having an
>> unrecoverable read error among all the drives at recover time starts
>> becoming significant.  Sure, you can use it - as long as a restore from
>> tape, etc. is an acceptable fallback if you can't rebuild after a drive
>> replacement.
>
> Actually, just 3 hours ago, I had a storage consultant telling me about
> a whitepaper that indicated a 20%/year chance of failure on a 5-disk
> RAID 5 of consumer grade SATA hard drives.  I can't find the paper he
> refers to, and the numbers seem high to me, but hey, I'm not going to
> chance it.
>
> David

Seems low to me for 1 TB drives.  And I've run the numbers with 2 TB
drives.  (I don't recall exactly what the results were off-hand.  But
they were scary.)

Simply don't use raid 5 with large drives if you don't want to lose
your array from time to time.

Greg