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. > 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 -- 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
