On Thu, Nov 01, 2018 at 02:49:03PM +0800, Ethan Lien wrote: > Snapshot is expected to be fast. But if there are writers steadily > create dirty pages in our subvolume, the snapshot may take a very long > time to complete. To fix the problem, we use tagged writepage for snapshot > flusher as we do in the generic write_cache_pages(), so we can ommit pages > dirtied after the snapshot command. > > We do a simple snapshot speed test on a Intel D-1531 box: > > fio --ioengine=libaio --iodepth=32 --bs=4k --rw=write --size=64G > --direct=0 --thread=1 --numjobs=1 --time_based --runtime=120 > --filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5; > time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio > > original: 1m58sec > patched: 6.54sec > > This is the best case for this patch since for a sequential write case, > we omit nearly all pages dirtied after the snapshot command. > > For a multi writers, random write test: > > fio --ioengine=libaio --iodepth=32 --bs=4k --rw=randwrite --size=64G > --direct=0 --thread=1 --numjobs=4 --time_based --runtime=120 > --filename=/mnt/sub/testfile --name=job1 --group_reporting & sleep 5; > time btrfs sub snap -r /mnt/sub /mnt/snap; killall fio > > original: 15.83sec > patched: 10.35sec > > The improvement is less compared with the sequential write case, since > we omit only half of the pages dirtied after snapshot command. > > Signed-off-by: Ethan Lien <ethanl...@synology.com>
This looks nice, thanks. I agree with the The suggestions from Nikolay, please update and resend. I was bit curious about the 'livelock', what you describe does not seem to be one. System under heavy IO can make the snapshot dead slow but can recover from that once the IO stops. Regarding the sync semantics, there's AFAIK no change to the current state where the sync is done before snapshot but without further other guarantees. From that point I think it's safe to select only subset of pages and make things faster. As the requested changes are not functional I'll add the patch to for-next for testing.
