Re: detection/correction of corruption with raid6

[Redeeman <redeeman@metanurb.dk>]

On Fri, 2008-12-05 at 22:30 +0100, 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.

thats a shame, it seems like a KILLER feature, but i guess its not too
simple to do, or it would have been done already :)

> 
> Greets,
> Mike
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