Re: Large latency with bcache for Ceph OSD

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On 2021/2/26 下午5:54, Coly Li wrote:
> On 2/26/21 4:57 PM, Norman.Kern wrote:
> [snipped]
>>> You may try to trigger a gc by writing to
>>> sys/fs/bcache/<cache-set-uuid>/internal/trigger_gc
>>>
>> When all cache had written back, I triggered gc, it recalled.
>>
>> root@WXS0106:~# cat /sys/block/bcache0/bcache/cache/cache_available_percent
>> 30
>>
>> root@WXS0106:~# echo 1 > /sys/block/bcache0/bcache/cache/internal/trigger_gc
>> root@WXS0106:~# cat /sys/block/bcache0/bcache/cache/cache_available_percent
>> 97
>>
>> Why must I trigger gc manually? Is not a default action of bcache-gc thread? And I found it can only work when all dirty data written back.
>>
> 1, GC is automatically triggered after some mount of data consumed. I
> guess it is just not about time in your situation.
>
> 2, Because the gc will shrink all cached clean data, which is very
> unfriendly for read-intend workload. Therefore gc_after_writeback is
> defaulted as 0, when this sysfs file content set to 1, a gc will trigger
> after the writeback accomplished.

I made a test again and get more infomation:

root@WXS0089:~# cat /sys/block/bcache0/bcache/dirty_data
0.0k
root@WXS0089:~# lsblk /dev/sda
NAME      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
sda         8:0    0 447.1G  0 disk
`-bcache0 252:0    0  10.9T  0 disk
root@WXS0089:~# cat /sys/block/sda/bcache/priority_stats
Unused:         1%
Clean:          29%
Dirty:          70%
Metadata:       0%
Average:        49
Sectors per Q:  29184768
Quantiles:      [1 2 3 5 6 8 9 11 13 14 16 19 21 23 26 29 32 36 39 43 48 53 59 65 73 83 94 109 129 156 203]
root@WXS0089:~# cat /sys/fs/bcache/066319e1-8680-4b5b-adb8-49596319154b/internal/gc_after_writeback
1
You have new mail in /var/mail/root
root@WXS0089:~# cat /sys/fs/bcache/066319e1-8680-4b5b-adb8-49596319154b/cache_available_percent
28

I read the source codes and found if cache_available_percent > 50, it should wakeup gc while doing writeback, but it seemed not work right.

>
> Coly Li
>
>
>
>




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