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

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

On Thu, Jan 31, 2013 at 01:12:20PM +0100, Wolfgang Denk wrote:
> Dear Piergiorgio,
> 
> In message <20130129202433.GB7005@lazy.lzy> you wrote:
> >
> > If all error report by raid6check, on the three
> > systems, are "unknown", then it seems to be a
> > software problem.
> 
> I think we can be pretty sure of this now.  For a test, I installed a
> vanilla mainline Linux kernel (v3.8-rc5) on the affected machines.
> 
> A "check" operation showed no more problems, but "raid6test"
> still reported a large number of errors like these:

Hi Wolfgang,

this surprise me quite a lot, the two checks should
have similar results. The only algorithmic difference
I know of is that raid6check reports "per stripe",
while the in kernel check should report "per block".
 
> ...
> P(4) wrong at 10291
> Q(5) wrong at 10291
> Error detected at 10291: disk slot unknown
> P(3) wrong at 10292
> Q(4) wrong at 10292
> Error detected at 10292: disk slot unknown
> P(2) wrong at 10293
> Q(3) wrong at 10293
> Error detected at 10293: disk slot unknown
> ...
> 
> After running a "repair" on the array, both "check" and "raid6test"
> would not report any further issues.

Which is again a surprise, if the repair changed
the parities, then the raid6check should complain,
if before it was not.
 
This confuses me a lot, I think Neil Brown or
H. Peter Anvin should comment on this situation.

bye.

-- 

piergiorgio