On 08/01/17 20:46, Piergiorgio Sartor wrote:

"should" as in "it is supposed to do it".

So, as far as I know, "raid6check" with "repair" will
check the parity and try to find errors.
If possible, it will find where the error is, then
re-compute the value and write the corrected data.

Now, this was somehow tested and *should* work.

An other option is just to check for the errors and
see if one drive is constantly at fault.
This will not write anything, so it is safer, but
it will help to see if there are strange things,
before writing to the disk(s).

Hmmm ...

I've now been thinking about it, and actually I'm not sure it's possible
even with raid6, to correct a corrupt read. The thing is, raid protects
against a failure to read - if a sector fails, the parity will re-create
it. But if a data sector is corrupted, how is raid to know WHICH sector?

If one of the parity sectors is corrupted, it's easy. Calculate parity
from the data, and either P or Q will be wrong, so fix it. But if it's a
*data* sector that's corrupted, both P and Q will be wrong. How easy is
it to work back from that, and work out *which* data sector is wrong? My
fu makes me think you can't, though I could quite easily be wrong :-)

But should that even happen, unless a disk is on its way out, anyway? I
remember years ago, back in the 80s, our minicomputers had
error-correction in the drive. I don't remember the algorithm, but it
wrote 16-bit words to disk - each an 8-bit data byte. The first half was
the original data, and the second half was some parity pattern such that
for any single-bit corruption you knew which half was corrupt, and you
could throw away the corrupt parity, or recreate the correct data from
the parity. Even with a 2-bit error I think it was >90% detection and
recreation. I can't imagine something like that not being in drive
hardware today.

Cheers,
Wol