Hi Robert, On Fri, Jul 20, 2012 at 12:53:10PM +0200, Robert Buchholz wrote: > Hello Piergiorgio, > > On Tuesday, July 03, 2012 10:27:34 PM Piergiorgio Sartor wrote: > > Hi Robert, > > > > On Tue, Jul 03, 2012 at 09:10:41PM +0200, Robert Buchholz wrote: > > [...] > > > > > > Why always two blocks? > > > > > > The reason is simply to have less cases to handle in the code. > > > There's already three ways to regenerate regenerate two blocks > > > (D&D, D/P&Q and D&P), and there would be two more cases if only > > > one block was to be repaired. With the original patch, if you can > > > repair two blocks, that allows you to repair one (and one other > > > in addition) as well. > > sorry, I express myself not clearly. > > > > I mean, a two parities Reed-Solomon system can > > only detect one incorrect slot position, so I would > > expect to have the possibility to fix only one, not > > two slots. > > > > So, I did not understand why two. I mean, I understand > > that a RAID-6 can correct exact up two incorrect slots, > > but the "unknown" case might have more and correcting > > will mean no correction or, maybe, even more damage. > > Well, if two slots have failed and you do not know which, or more than > two have failed, there is no way to recover anything reliably. > I implemented the two slot fix to recover from a case where you *do* > know which two slots failed (e.g., from syslog messages such as this: > end_request: I/O error, dev sdk, sector 3174422). Obviously, this > expects a lot of knowledge from the admin running the command and > selecting the slots and comes with no guarantees that the "repaired" > blocks will contain more of the expected data than before. OK, thanks, I see. > > I would prefer, if you agree, to simply tell "raid6check" > > to fix a single slot, or the (single) wrong slots it finds > > during the check. > > > > Does it make sense to you, or, maybe, you're considering > > something I'm missing? > > This makes perfect sense. > > > > > Of course, this is just a statistical assumption, which > > > > means a second, "aggressive", option will have to be > > > > available, with all the warnings of the case. > > > > > > As you point out, it is impossible to determine which of two > > > failed > > > slots are in error. I would leave such decision to an admin, but > > > giving one or more "advices" may be a nice idea. > > > > That would be exactly the background. > > For example, considering that "raid6check" processes > > stripes, but the check is done per byte, already > > knowing how many bytes per stripe (or block) need > > to be corrected (per device) will hint a lot about > > the overall status of the storage. > > That piece of information is definitely interesting. What is the > smartest way to determine the number of incorrect bytes for one failed > slot? The function "raid6_collect()" of "raif6check" performs the check per byte position and returns an array (of int) in which each value has a status representing the condition of the bytes of the slot at that position. The possible values are, if my memory serves me: -255 = OK positive integer = failed data (if greater than number of disks, then "unknown", of course, but this is checked later) -1 or -2 = failed P or Q parity The "raid6_stats()" function uses the returned array in order to try to detect the status of the complete slot. This (bugs apart) tries to be consistent, that is different errors are considered "unknown" slot status. But this is the minimal possible approach. Now, the "results[]" array could also be used to count how many positions of the slot are correct or incorrect and how. I guess this information could be used to understand better the status of the slot. Collecting statistics across the slots, that is for the whole array, could allow to do a better assessment of the RAID-6. It clearly makes a difference if a single byte position is incorrect or all. Or if a byte is -1 and all the others are -2. > > > Personally, I am recovering from a simultaneous three-disk failure > > > on a backup storage. My best hope was to ddrescue "most" from all > > > three disks onto fresh ones, and I lost a total of a few KB on > > > each disk. Using the ddrescue log, I can even say which sectors > > > of each disk were damaged. Interestingly, two disks of the same > > > model failed on the very same sector (even though they were > > > produced at different times), so I now have "unknown" slot errors > > > in some stripes. But with context information, I am certain I > > > know which slots need to be repaired. > > That's good! > > Did you use "raid6check" for a verification? > > Yes, since John Robinson pointed me to it earlier in this thread. > > > > I am a big supporter of getting it to work, then make it fast. > > > Since a full raid check takes the magnitude of hours anyway, I do > > > not mind that repairing blocks from the user space will take five > > > minutes when it could be done in 3. That said, I think the faster > > > code in the kernel is warranted (as it needs this calculation > > > very often when a disk is failed), and if it is possible to reuse > > > easily, we sure should. > > The check is pretty slow, also due to the terminal > > print out, which is a bit too verbose, I think. > > That is true. The stripe geometry output could be optional, especially > when there is no error to be reported. > > > Anyhow, I'm really happy someone has interest in > > improving "raid6check", I hope you'll be able to > > improve it and, maybe, someone else will join > > the bandwagon... :-) > > Well, thank you for starting it and sorry for my slow replies. > > > Cheers > > Robert bye, -- piergiorgio -- 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
