On 12/19/20 03:28, Kent Overstreet wrote:
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.
Hi Kent and Coly,
Bcachefs, I'm glad to hear that and keen on having a trial, thank you for all your great work!
Pardon my ignorance I just started reading bcache codes and they are not easy to understand,
so as per Kent, is it possible to port things using in bcachefs to make I/O hang avoided
completely in bcache?
Since bcachefs is a relavite new(but great) filesystem and it needs some time to be matured,
if we can kill GC IO-stall completely in bcache, users won't be suffered from filesystem
migration while using bcache.
linfeng
