Re: Questions about bitrot and RAID 5/6

[Phil Turmel <philip@turmel.org>]

Hi Chris,

On 01/23/2014 12:28 PM, Chris Murphy wrote:
> It's a fair point. I've recently run across some claims on a separate
> forum with hardware raid5 arrays containing all enterprise drives,
> with regularly scrubs, yet with such excessive implosions that some
> integrators have moved to raid6 and completely discount the use of
> raid5. The use case is video production. This sounds suspiciously
> like microcode or raid firmware bugs to me. I just don't see how ~6-8
> enterprise drives in a raid5 translates into significantly higher
> array collapses that then essentially vanish when it's raid6.

I just wanted to address this one point.  Raid6 is many orders of
magnitude more robust than raid5 in the rebuild case.  Let me illustrate:

How to lose data in a raid5:

1) Experience unrecoverable read errors on two of the N drives at the
same *time* and same *sector offset* of the two drives.  Absurdly
improbable.  On the order of 1x10^-36 for 1T consumer-grade drives.

2a) Experience hardware failure on one drive followed by 2b) an
unrecoverable read error in another drive.  You can expect a hardware
failure rate of a few percent per year.  Then, when rebuilding on the
replacement drive, the odds skyrocket.  On large arrays, the odds of
data loss are little different from the odds of a hardware failure in
the first place.

How to lose data in a raid6:

1) Experience unrecoverable read errors on *three* of the N drives at
the same *time* and same *sector offset* of the drives.  Even more
absurdly improbable.  On the order of 1x10^-58 for 1T consumer-grade drives.

2) Experience hardware failure on one drive followed by unrecoverable
read errors on two of the remaining drives at the same *time* and same
*sector offset* of the two drives.  Again, absurdly improbable.  Same as
for the raid5 case "1".

3) Experience hardware failure on two drives followed by an
unrecoverable read error in another drive.  As with raid5 on large
arrays, you probably can't complete the rebuild error-free.  But the
odds of this event are subject to management--quick reponse to case "2"
greatly reduces the odds of case "3".

It is no accident that raid5 is becoming much less popular.

Phil