Re: detection/correction of corruption with raid6

[Peter Rabbitson <rabbit+list@rabbit.us>]

Michał Przyłuski wrote:
> Hi,
> 
> 2008/12/5 Redeeman <redeeman@metanurb.dk>:
>> On Fri, 2008-12-05 at 16:09 -0500, Justin Piszcz wrote:
>>> On Fri, 5 Dec 2008, Redeeman wrote:
>>>
>>>> On Fri, 2008-12-05 at 16:02 -0500, Justin Piszcz wrote:
>>>>> On Fri, 5 Dec 2008, Redeeman wrote:
>>>>>
>>>>>> Hello.
>>>>>>
>>>>>> I was looking at the PDFs linked to from the wiki, and found this:
>>>>>> http://kernel.org/pub/linux/kernel/people/hpa/raid6.pdf
>>>>>>
>>>>>> More specifically, section 4, starting on page 8.
>>>>>>
>>>>>> Am I understanding this correctly, in that with raid6, linux is capable
>>>>>> of detecting if the content on 1 disk is corrupted, and reconstruct it
>>>>>> from the remaining disks?
>>>>> I ran md/raid6 for awhile, do you mean remap the bad sector on the fly?
>>>>> Linux/md raid does not do this afaik.
>>>> No, i mean, if one disk does silent corruption
>>> What would the error look like?  Both md/Linux & in the 3ware manual
>>> recommend you run a 'check' across the raid at least once a week
>>> (3ware/raid-verify) and md/Linux in Debian runs a check once a month I
>>> believe to eliminate these issues.
>>>
>>> If you are asking whether a read error of a latent sector from the one
>>> disk will result it reading the data from the second disk that is a good
>>> question.
>> im asking, if one disk in a raid6 setup suddenly decides to flip a few
>> bits in some bytes, will it be able to detect that in a scan, and
>> correct it? i cant see how it can do it on raid5, but maybe raid6?
> 
> No, not really.
> I've been investigating silent corruption for a quite a while now, and
> it looks more or less like this.
> During a "check" action it'll be detected. During normal operation -
> it won't be detected.
> Normal (non-degraded) raid5/6 reads don't read parity (or Q syndrome),
> they just read data. So they have no idea that something went bad.
> Now, worse news is that you cannot really fix it automagically, even
> after detecting by a "check" procedure. A "repair" will overwrite
> parity and Q syndrome, with new values (new = calculated from what it
> seems to be data blocks).
> 
> It is possible (by the theory of Q syndrome, per the article you
> linked) to detect which drive is doing a silent corruption with raid6
> (and with some extra assumption, that just one drive is doing that).
> But it's not implemented.
> 

I'd like to shamelessly bring in an older related thread:
http://marc.info/?l=linux-raid&m=120605458309825
http://marc.info/?l=linux-raid&m=120618020817057

Maybe someone will get inspired, and will actually write the damned thing :)

Cheers
Peter
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