On Tue 03-06-14 07:14:44, Christoph Hellwig wrote: > On Tue, Jun 03, 2014 at 04:05:31PM +0200, Jan Kara wrote: > > So we currently flush inodes in first dirtied first written back order when > > superblock is not specified in writeback work. That completely ignores the > > fact to which superblock inode belongs but I don't see per-sb fairness to > > actually make any sense when > > 1) flushing old data (to keep promise set in dirty_expire_centisecs) > > 2) flushing data to reduce number of dirty pages > > And these are really the only two cases where we don't do per-sb flushing. > > > > Now when filesystems want to do something more clever (and I can see > > reasons for that e.g. when journalling metadata, even more so when > > journalling data) I agree we need to somehow implement the above two types > > of writeback using per-sb flushing. Type 1) is actually pretty easy - just > > tell each sb to writeback dirty data upto time T. Type 2) is more difficult > > because that is more openended task - it seems similar to what shrinkers do > > but that would require us to track per sb amount of dirty pages / inodes > > and I'm not sure we want to add even more page counting statistics... > > Especially since often bdi == fs. Thoughts? > > Honestly I think doing per-bdi writeback has been a major mistake. As > you said it only even matters when we have filesystems on multiple > partitions on a single device, and even then only in a simple setup, > as soon as we use LVM or btrfs this sort of sharing stops to happen > anyway. I don't even see much of a benefit except that we prevent > two flushing daemons to congest a single device for that special case > of multiple filesystems on partitions of the same device, and that could > be solved in other ways. So I agree per-bdi / per-sb matters only in simple setups but machines with single rotating disk with several partitions and without LVM aren't that rare AFAICT from my experience. And I agree we went for per-bdi flushing to avoid two threads congesting a single device leading to suboptimal IO patterns during background writeback. So currently I'm convinced we want to go for per-sb dirty tracking. That also makes some speedups in that code noticeably simpler. I'm not convinced about the per-sb flushing thread - if we don't regress the multiple sb on bdi case when we just let the threads from different superblocks contend for IO, then that would be a natural thing to do. But once we have to introduce some synchronization between threads to avoid regressions, I think it might be easier to just stay with per-bdi thread which switches between superblocks. Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
