Tejun Heo <tj@xxxxxxxxxx> writes: > Hello, > > On Tue, Aug 02, 2011 at 01:39:46PM -0400, Jeff Moyer wrote: >> OK, sorry for top-posting here, but I chased the problem down further. >> >> Commit ae1b1539622fb46e51b4d13b3f9e5f4c713f86ae, block: reimplement >> FLUSH/FUA to support merge, introduced a regression when running any >> sort of fsyncing workload using dm-multipath and certain storage (in our >> case, an HP EVA). It turns out that dm-multipath always advertised >> flush+fua support, and passed commands on down the stack, where they >> used to get stripped off. The above commit, unfortunately, changed that >> behavior: > ... >> So, the flush machinery was bypassed in such cases (q->flush_flags == 0 >> && rq->cmd_flags & (REQ_FLUSH|REQ_FUA)). >> >> Now, however, we don't get into the flush machinery at all (which is why >> my initial patch didn't help this situation). Instead, >> __elv_next_request just hands a request with flush and fua bits set to >> the scsi_request_fn, even though the underlying request_queue does not >> support flush or fua. >> >> So, where do we fix this? We could just accept Mike's patch to not send >> such requests down from dm-mpath, but that seems short-sighted. We >> could reinstate some checks in __elv_next_request. Or, we could put the >> checks into blk_insert_cloned_request. > > Ah, okay, what changed there was where a request is passed into flush > machinery. Before, it was while the request was being dispatched from > elevator to device. After, it's de-composed when the request enters > elevator. The bug is that there are paths which insert new requests > to elevator but didn't check for REQ_FLUSH|FUA. > > I think it would be cleaner to add a wrapper around > __elv_add_request() which checks for REQ_FLUSH|FUA and enforce > REQ_INSERT_FLUSH if the request needs it. Note that this should only > happen when a request enters the queue for the first time but not on > requeues - that was the reason why the decision wasn't made inside > __elv_add_request(). OK, we can do a wrapper, but it probably wouldn't be too horrific to just fix up blk_insert_cloned_request. Now, the next issue is that flush requests issued from the dm target down through the stack have no bio associated with them, so we blow up on the BUG_ON(!req->bio || req->bio != req->biotail). Cheers, Jeff -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
