On Fri, 2008-12-05 at 22:30 +0100, Michał Przyłuski wrote: > Hi, > > 2008/12/5 Redeeman <redeeman@xxxxxxxxxxx>: > > 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
