NeilBrown <neilb@xxxxxxx> wrote: >On Fri, 22 Jul 2011 18:10:33 +0900 Namhyung Kim <namhyung@xxxxxxxxx> >wrote: > >> NeilBrown <neilb@xxxxxxx> writes: >> >> > RAID6 is only allowed to choose 'reconstruct-write' while RAID5 is >> > also allow 'read-modify-write' >> > Apart from this difference, handle_stripe_dirtying[56] are nearly >> > identical. So resolve these differences and create just one >function. >> > >> > Signed-off-by: NeilBrown <neilb@xxxxxxx> >> >> Reviewed-by: Namhyung Kim <namhyung@xxxxxxxxx> >> >> BTW, here is a question: >> Why RAID6 doesn't allow the read-modify-write? I don't think it is >> not possible, so what prevents doing that? performance? complexity? >> or it's not possible? Why? :) > > >The primary reason in my mind is that when Peter Anvin wrote this code >he >didn't implement read-modify-write and I have never seen a need to >consider >changing that. > >You would need a largish array - at least 7 devices - before RWM could >possibly be a win, but some people do have arrays larger than that. > >The computation to "subtract" from the Q-syndrome might be a bit >complex - I >don't know. > >Peter: maybe you have a reason that I haven't mentioned? > >Thanks, >NeilBrown When I measured it it wasn't a win on my system, and it created a lot of extra complexity. It probably would be a win with SSSE3 and lots of drives. -- Sent from my mobile phone. Please excuse my brevity and lack of formatting. -- 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
