From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Rabbitson Subject: Re: detection/correction of corruption with raid6 Date: Fri, 05 Dec 2008 23:12:44 +0100 Message-ID: <4939A75C.6050705@rabbit.us> 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=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <6c4602af0812051330o4220cbf1g8b969cd4b9843d3a@mail.gmail.com> Sender: linux-raid-owner@vger.kernel.org To: =?UTF-8?B?TWljaGHFgiBQcnp5xYJ1c2tp?= Cc: Redeeman , linux-raid@vger.kernel.org List-Id: linux-raid.ids Micha=C5=82 Przy=C5=82uski 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 thi= s: >>>>>> 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 reconstr= uct it >>>>>> from the remaining disks? >>>>> I ran md/raid6 for awhile, do you mean remap the bad sector on th= e 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 manua= l >>> 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 mont= h 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 f= ew >> 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. >=20 I'd like to shamelessly bring in an older related thread: http://marc.info/?l=3Dlinux-raid&m=3D120605458309825 http://marc.info/?l=3Dlinux-raid&m=3D120618020817057 Maybe someone will get inspired, and will actually write the damned thi= ng :) Cheers Peter -- 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