Hi Tejun, On 08/25/2010 01:59 AM +0900, Tejun Heo wrote: > On 08/24/2010 12:24 PM, Kiyoshi Ueda wrote: >> Anyway, only reporting errors for REQ_FLUSH to upper layer without >> such a solution would make dm-multipath almost unusable in real world, >> although it's better than implicit data loss. > > I see. > >>> Maybe just turn off barrier support in mpath for now? >> >> If it's possible, it could be a workaround for a short term. >> But how can you do that? >> >> I think it's not enough to just drop REQ_FLUSH flag from q->flush_flags. >> Underlying devices of a mpath device may have write-back cache and >> it may be enabled. >> So if a mpath device doesn't set REQ_FLUSH flag in q->flush_flags, it >> becomes a device which has write-back cache but doesn't support flush. >> Then, upper layer can do nothing to ensure cache flush? > > Yeah, I was basically suggesting to forget about cache flush w/ mpath > until it can be fixed. You're saying that if mpath just passes > REQ_FLUSH upwards without retrying, it will be almost unuseable, > right? Right. If the error is safe/needed to retry using other paths, mpath should retry even if REQ_FLUSH. Otherwise, only one path failure may result in system down. Just passing any REQ_FLUSH error upwards regardless the error type will make such situations, and users will feel the behavior as unstable/unusable. > I'm not sure how to proceed here. How much work would > discerning between transport and IO errors take? If it can't be done > quickly enough the retry logic can be kept around to keep the old > behavior but that already was a broken behavior, so... :-( I'm not sure how long will it take. Anyway, as you said, the flush error handling of dm-mpath is already broken if data loss really happens on any storage used by dm-mpath. Although it's a serious issue and quick fix is required, I think you may leave the old behavior in your patch-set, since it's a separate issue. Thanks, Kiyoshi Ueda -- 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
