Re: Huge values of mismatch_cnt on RAID 6 arrays under Fedora 18

[Wolfgang Denk <wd@denx.de>]

Dear Chris,

In message <ADB1B276-21F3-4006-A613-F979F931EDC3@colorremedies.com> you wrote:
> 
> > Correct, these are 3 different machines.
> 
> Too bad. Better to test first, than commit so many computers and arrays
> for such a major change.

In hindsight you are of course correct.  But then, these are still not
really vitally critically systems, and I hve to admit that I did not
expect such kind of problems.  I have installed a large number of
Fedora releases before (all of them since FC4 actually, on quite a
number of systems), and while there have always been some problems, I
never ran into something like this before.

> Unclear. If parity chunks are both wrong, then that means you
> effectively have partial RAID 0 depending on what parity chunks are
> correct or not. I'm not recommending this, but if you set one disk to
> faulty and started your file system and file tests again… if they're
> bad then indeed it's parity that's affected. If you don't get errors,
> then it indicates the test method is insufficient to locate the errors
> and it could still be data that's affected.

OK, I will keep this in mind. If needed, I can dedicate one of the
systems to even a destructive test without too much actual loss.

> It's a tenuous situation. It might be wise to pick a low priority
> computer for regression, and hopefully the problem gets better rather
> than worse. If the assumption is that the parity is bad, it needs to be
> recalculated with repair. If that goes well with tests and another check
> scrub, then it's better to get on with additional regressions sooner
> than later. Again in the meantime if you lost a drive, it could be a
> real mess if the raid starts to rebuild bad data from parity. Or even
> starts to write user data incorrectly too.

Well, I did this - the repair worked without errors, but it left again
a huge mismatch_cnt; raid6check on this array has not found any
problems so far - even though I see mismatch_cnt = 362731480

Best regards,

Wolfgang Denk

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