On Sun, Jan 08, 2017 at 09:06:14PM +0000, Wols Lists wrote: > On 08/01/17 20:46, Piergiorgio Sartor wrote: > > "should" as in "it is supposed to do it". > > > > So, as far as I know, "raid6check" with "repair" will > > check the parity and try to find errors. > > If possible, it will find where the error is, then > > re-compute the value and write the corrected data. > > > > Now, this was somehow tested and *should* work. > > > > An other option is just to check for the errors and > > see if one drive is constantly at fault. > > This will not write anything, so it is safer, but > > it will help to see if there are strange things, > > before writing to the disk(s). > > Hmmm ... > > I've now been thinking about it, and actually I'm not sure it's possible > even with raid6, to correct a corrupt read. The thing is, raid protects > against a failure to read - if a sector fails, the parity will re-create > it. But if a data sector is corrupted, how is raid to know WHICH sector? Here all you need to know: http://ftp.nluug.nl/ftp/ftp/os/Linux/system/kernel/people/hpa/raid6.pdf bye, pg > > If one of the parity sectors is corrupted, it's easy. Calculate parity > from the data, and either P or Q will be wrong, so fix it. But if it's a > *data* sector that's corrupted, both P and Q will be wrong. How easy is > it to work back from that, and work out *which* data sector is wrong? My > fu makes me think you can't, though I could quite easily be wrong :-) > > But should that even happen, unless a disk is on its way out, anyway? I > remember years ago, back in the 80s, our minicomputers had > error-correction in the drive. I don't remember the algorithm, but it > wrote 16-bit words to disk - each an 8-bit data byte. The first half was > the original data, and the second half was some parity pattern such that > for any single-bit corruption you knew which half was corrupt, and you > could throw away the corrupt parity, or recreate the correct data from > the parity. Even with a 2-bit error I think it was >90% detection and > recreation. I can't imagine something like that not being in drive > hardware today. > > Cheers, > Wol -- 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
