From mboxrd@z Thu Jan 1 00:00:00 1970 From: Redeeman Subject: Re: detection/correction of corruption with raid6 Date: Fri, 12 Dec 2008 16:31:24 +0100 Message-ID: <1229095884.16555.144.camel@localhost> References: <1228510833.16555.76.camel@localhost> <1228511195.16555.78.camel@localhost> <1228511558.16555.84.camel@localhost> <6c4602af0812051330o4220cbf1g8b969cd4b9843d3a@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-2 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <6c4602af0812051330o4220cbf1g8b969cd4b9843d3a@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: =?iso-8859-2?Q?Micha=B3_Przy=B3uski?= Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Fri, 2008-12-05 at 22:30 +0100, Micha=B3 Przy=B3uski wrote: > Hi, >=20 > 2008/12/5 Redeeman : > > 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 t= his: > >> >>> 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 i= s capable > >> >>> of detecting if the content on 1 disk is corrupted, and recons= truct 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 manu= al > >> 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 mon= th 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? >=20 > No, not really. > I've been investigating silent corruption for a quite a while now, an= d > 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 =3D calculated from what = it > seems to be data blocks). >=20 > 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 :) >=20 > Greets, > Mike > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid"= in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html