On Fri, Dec 18, 2020 at 07:52:10PM +0800, Coly Li wrote: > On 12/18/20 6:35 PM, Lin Feng wrote: > > Hi all, > > > > I googled a lot but only finding this, my question is if this issue have > > been fixed or > > if there are ways to work around? > > > >> On Wed, 28 Jun 2017, Coly Li wrote: > >> > >> > On 2017/6/27 下午8:04, tang.junhui@xxxxxxxxxx wrote: > >> > > Hello Eric, Coly, > >> > > > >> > > I use a 1400G SSD device a bcache cache device, > >> > > and attach with 10 back-end devices, > >> > > and run random small write IOs, > >> > > when gc works, It takes about 15 seconds, > >> > > and the up layer application IOs was suspended at this time, > >> > > How could we bear such a long time IO stopping? > >> > > Is there any way we can avoid this problem? > >> > > > >> > > I am very anxious about this question, any comment would be valuable. > >> > > >> > I encounter same situation too. > >> > Hmm, I assume there are some locking issue here, to prevent application > >> > to send request and insert keys in LSM tree, no matter in writeback or > >> > writethrough mode. This is a lazy and fast response, I need to check > > the > >> > code then provide an accurate reply :-) > >> > > > > I encoutered even worse situation(8TB ssd cached for 4*10 TB disks) as > > mail extracted above, > > all usrer IOs are hung during bcache GC runs, my kernel is 4.18, while I > > tested it with kernel 5.10, > > it seems that situation is unchaged. > > > > Below are some logs for reference. > > GC trace events: > > [Wed Dec 16 15:08:40 2020] ##48735 [046] .... 1632697.784097: > > bcache_gc_start: 4ab63029-0c4a-42a8-8f54-e638358c2c6c > > [Wed Dec 16 15:09:01 2020] ##48735 [034] .... 1632718.828510: > > bcache_gc_end: 4ab63029-0c4a-42a8-8f54-e638358c2c6c > > > > and during which iostat shows like: > > 12/16/2020 03:08:48 PM > > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s > > avgrq-sz avgqu-sz await r_await w_await svctm %util > > sdb 0.00 0.50 1325.00 27.00 169600.00 122.00 > > 251.07 0.32 0.24 0.24 0.02 0.13 17.90 > > sdc 0.00 0.00 0.00 0.00 0.00 0.00 > > 0.00 0.00 0.00 0.00 0.00 0.00 0.00 > > sdd 0.00 0.00 0.00 0.00 0.00 0.00 > > 0.00 0.00 0.00 0.00 0.00 0.00 0.00 > > sde 0.00 0.00 0.00 0.00 0.00 0.00 > > 0.00 0.00 0.00 0.00 0.00 0.00 0.00 > > sdf 0.00 0.00 0.00 0.00 0.00 0.00 > > 0.00 0.00 0.00 0.00 0.00 0.00 0.00 > > bcache0 0.00 0.00 1.00 0.00 4.00 0.00 > > 8.00 39.54 0.00 0.00 0.00 1000.00 100.00 > > > > # grep . /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/*gc* > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/btree_gc_average_duration_ms:26539 > > > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/btree_gc_average_frequency_sec:8692 > > > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/btree_gc_last_sec:6328 > > > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/btree_gc_max_duration_ms:283405 > > > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/copy_gc_enabled:1 > > > > /sys/fs/bcache/4ab63029-0c4a-42a8-8f54-e638358c2c6c/internal/gc_always_rewrite:1 > > I/O hang during GC is as-designed. We have plan to improve, but the I/O > hang cannot be 100% avoided. This is something that's entirely fixed in bcachefs - we update bucket sector counts as keys enter/leave the btree so runtime btree GC is no longer needed.
