On Wed, 28 Apr 2010 01:02:14 +0200 MRK <mrk@xxxxxxxxxxxxx> wrote: > On 04/27/2010 05:50 PM, Janos Haar wrote: > > > > ----- Original Message ----- From: "MRK" <mrk@xxxxxxxxxxxxx> > > To: "Janos Haar" <janos.haar@xxxxxxxxxxxx> > > Cc: <linux-raid@xxxxxxxxxxxxxxx> > > Sent: Monday, April 26, 2010 6:53 PM > > Subject: Re: Suggestion needed for fixing RAID6 > > > > > >> On 04/26/2010 02:52 PM, Janos Haar wrote: > >>> > >>> Oops, you are right! > >>> It was my mistake. > >>> Sorry, i will try it again, to support 2 drives with dm-cow. > >>> I will try it. > >> > >> Great! post here the results... the dmesg in particular. > >> The dmesg should contain multiple lines like this "raid5:md3: read > >> error corrected ....." > >> then you know it worked. > > > > I am affraid i am still right about that.... > > > > ... > > end_request: I/O error, dev sdh, sector 1667152256 > > raid5:md3: read error not correctable (sector 1662188168 on dm-1). > > raid5: Disk failure on dm-1, disabling device. > > raid5: Operation continuing on 10 devices. > > I think I can see a problem here: > You had 11 active devices over 12 when you received the read error. > At 11 devices over 12 your array is singly-degraded and this should be > enough for raid6 to recompute the block from parity and perform the > rewrite, correcting the read-error, but instead MD declared that it's > impossible to correct the error, and dropped one more device (going to > doubly-degraded). > > I think this is an MD bug, and I think I know where it is: > > > --- linux-2.6.33-vanilla/drivers/md/raid5.c 2010-02-24 > 19:52:17.000000000 +0100 > +++ linux-2.6.33/drivers/md/raid5.c 2010-04-27 23:58:31.000000000 +0200 > @@ -1526,7 +1526,7 @@ static void raid5_end_read_request(struc > > clear_bit(R5_UPTODATE, &sh->dev[i].flags); > atomic_inc(&rdev->read_errors); > - if (conf->mddev->degraded) > + if (conf->mddev->degraded == conf->max_degraded) > printk_rl(KERN_WARNING > "raid5:%s: read error not correctable " > "(sector %llu on %s).\n", > > ------------------------------------------------------ > (This is just compile-tested so try at your risk) > > I'd like to hear what Neil thinks of this... I think you've found a real bug - thanks. It would make the test '>=' rather than '==' as that is safer, otherwise I agree. > - if (conf->mddev->degraded) > + if (conf->mddev->degraded >= conf->max_degraded) Thanks, NeilBrown > > The problem here (apart from the erroneous error message) is that if > execution goes inside that "if" clause, it will eventually reach the > md_error() statement some 30 lines below there, which will have the > effect of dropping one further device further worsening the situation > instead of recovering it, and this is not the correct behaviour in this > case as far as I understand. > At the current state raid6 behaves like if it was a raid5, effectively > supporting only one failed disk. > > -- > 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
