Re: detection/correction of corruption with raid6

[Redeeman <redeeman@metanurb.dk>]

On Tue, 2008-12-16 at 22:58 +0100, Piergiorgio Sartor wrote:
> Hi all,
> 
> while I do agree that the issue needs more in deep thinking,
> I would like to tell a recent story that happened to me.
> 
> I was testing a RAID-6 array, with 7, small, HDs.
> Intention was to get used to different situations, repair,
> grow, fail, remove, etc.
> 
> After some playing, I started to check the files on the array
> and I found out that they were not (always) correct.
> So I started a check of the array, which returned some 1000 or
> more mismatches.
> 
> After some investigation, I found out that one HD had a "flaky"
> interface, data was correctly written, but sometimes, randomly,
> reading returned some "wrong" bits (re-cabling solved the issue).
> 
> To check this with RAID-6, I could run the check with 6 disks,
> for 7 times, each with a different disk removed, until one run
> returned no mismatches.
> At this point, I knew which "data path" was defective.
> 
> It would have saved a lot of time, if the check could have
> done this automatically...

Exactly! this is partly the point i make too

> 
> So, my RFE, would be, if possible, to try, during RAID-6 check,
> to find out if and which HD has the mismatch.
> Ideally, at the end of the check, the system log should show
> how many mismatches, if any, are likely to belong to which HD
> or are undetermined.
> This would help to diagnose the full data path and reduce
> testing time in case of problems.
> In case only one HD results problematic, this one could be
> failed, removed and the complete cabling, I/F and so on checked.
> Of course, this goes beyond the simple "HD failure protection"
> scope of RAID, nevertheless I do not see why this possibility
> should be neglected, unless it is too complex/difficult to
> implement and maintain.
Yeah, I myself do not know how much more complicated this would make
things, but i would imagine it would be worth it..
> 
> Regarding the possibility of recovery, I have one question:
> 
> Why a RAID system might have inconsistencies?
> Why do we have a "check" command at all, to run weekly or monthly?
As previously stated in discussion, while most bitflips etc does not
happen on disk(apparently), they do happen, whether its in ram, pci,
controller etc...

Also, i imagine its just to be on top of things, read and ensure stuff
works.. (but this is pure speculation)
> 
> Thanks,
> 
> bye,
> 
>