Defects about bcache GC

[Lin Feng <linf@wangsu.com>]

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

Thanks and Best wishes!
linfeng