On Mon, Nov 02, 2015 at 09:22:15AM -0500, Jeff Moyer wrote: > Dave Chinner <david@xxxxxxxxxxxxx> writes: > > > Further, REQ_FLUSH/REQ_FUA are more than just "put the data on stable > > storage" commands. They are also IO barriers that affect scheduling > > of IOs in progress and in the request queues. A REQ_FLUSH/REQ_FUA > > IO cannot be dispatched before all prior IO has been dispatched and > > drained from the request queue, and IO submitted after a queued > > REQ_FLUSH/REQ_FUA cannot be scheduled ahead of the queued > > REQ_FLUSH/REQ_FUA operation. > > > > IOWs, REQ_FUA/REQ_FLUSH not only guarantee data is on stable > > storage, they also guarantee the order of IO dispatch and > > completion when concurrent IO is in progress. > > This hasn't been the case for several years, now. It used to work that > way, and that was deemed a big performance problem. Since file systems > already issued and waited for all I/O before sending down a barrier, we > decided to get rid of the I/O ordering pieces of barriers (and stop > calling them barriers). > > See commit 28e7d184521 (block: drop barrier ordering by queue draining). Yes, I realise that, even if I wasn't very clear about how I wrote it. ;) Correct me if I'm wrong: AFAIA, dispatch ordering (i.e. the "IO barrier") is still enforced by the scheduler via REQ_FUA|REQ_FLUSH -> ELEVATOR_INSERT_FLUSH -> REQ_SOFTBARRIER and subsequent IO scheduler calls to elv_dispatch_sort() that don't pass REQ_SOFTBARRIER in the queue. IOWs, if we queue a bunch of REQ_WRITE IOs followed by a REQ_WRITE|REQ_FLUSH IO, all of the prior REQ_WRITE IOs will be dispatched before the REQ_WRITE|REQ_FLUSH IO and hence be captured by the cache flush. Hence once the filesystem has waited on the REQ_WRITE|REQ_FLUSH IO to complete, we know that all the earlier REQ_WRITE IOs are on stable storage, too. Hence there's no need for the elevator to drain the queue to guarantee completion ordering - the dispatch ordering and flush/fua write semantics guarantee that when the flush/fua completes, all the IOs dispatch prior to that flush/fua write are also on stable storage... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
