On Tue, Aug 24 2010 at 12:59pm -0400, Tejun Heo <tj@xxxxxxxxxx> wrote: > Hello, > > On 08/24/2010 12:24 PM, Kiyoshi Ueda wrote: > > Yes, checking whether it's a transport error in lower layer is > > the right solution. > > (Since I know it's not available yet, I just hoped if upper layers > > had some other options.) > > > > 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? I'm not sure how to proceed here. Seems clear that we must fix mpath to receive the SCSI errors, in some form, so it can decide if a retry is required/valid or not. Such error processing was a big selling point for the transition from bio-based to request-based multipath; so it's unfortunate that this piece has been left until now. > How much work would discerning between transport and IO errors take? Hannes already proposed some patches: https://patchwork.kernel.org/patch/61282/ https://patchwork.kernel.org/patch/61283/ https://patchwork.kernel.org/patch/61596/ This work was discussed at LSF, see "Error Handling - Hannes Reinecke" here: http://lwn.net/Articles/400589/ I thought James, Alasdair and others offered some guidance on what he'd like to see... Unfortunately, even though I was at this LSF session, I can't recall any specific consensus on how Hannes' work should be refactored (to avoid adding SCSI sense processing code directly in dm-mpath). Maybe James, Hannes or others remember? Was it enough to just have the SCSI sense processing code split out in a new sub-section of the SCSI midlayer -- and then DM calls that code? > 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'll have to review this thread again to understand why mpath's existing retry logic is broken behavior. mpath is used with more capable SCSI devices so I'm missing why a failed FLUSH implies data loss. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
