On Tue, Jan 29, 2013 at 12:18:39AM +0100, Wolfgang Denk wrote:
[...]

This is what I think at the moment, as all my samples of data so fare
looked ok.

Hi Wolfgang,

my personal opinion would be to confirm
*all* the data is OK, if you can.
This will point to the parity calculation
as error source, I guess.

quoted

This could be in case of some software bug, which
would be quite a surprise, I must say.

Indeed.  Guess how I feel...

The lucky one... :-)
 

quoted

Still, would be nice to check if the whole array
is it this state or if, sooner or later, some
knwon slot (with error) is found somewhere else.

Checks still running.  I see two things:

- on the array where I was running "repair" before, raid6check reports
  no errors so far - but still there is a mismatch_cnt = 362731480
  raid6check is still running.

As mentioned, the "repair" reports the
number of repairs it did, so unless
you ran a check after than, the number
is expected, I guess.
 

- on the second machine, I have 558579 lines out output, 176 of which
  are errors of type "Error detected at : disk slot unknown"; no other
  errors reported so far.  raid6check is still running.

Nah, ja, it is slow, I know...
In any case, as wrote in another post, "unknown"
means both parities are wrong and a suitable,
guilty, slot cannot be found.
So, either both parities are wrong and only
them (best case scenario), or more than one
disk has corruped data on the same stripe.

- on the third machine, I have 5512894 lines out output, 1599431 of
  which are errors of type "Error detected at : disk slot unknown"; no
  other errors reported so far.  raid6check is still running.

This smells really bad as if parity computation was broken...

Uhm, as mentioned, it would be nice to
find a specific error slot...
Well, not so nice, but this would point
to an HW problem.
 

OK, add more hardware details...

A: Supermicro X8SAX mainboard, Core i7 CPU 950 @ 3.07GHz, 24 GB RAM
H: Supermicro X8ST3 mainboard, Xeon CPU W3565  @ 3.20GHz, 24 GB RAM
X: Supermicro X8SAX mainboard, Core i7 CPU 950 @ 3.07GHz, 24 GB RAM

 
What does the kernel log says about the choosen
RAID6 algorithm?

There should be some information with "dmesg".

bye,

-- 

piergiorgio