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Subject: Re: Large latency with bcache for Ceph OSD
To:     "Norman.Kern" <norman.kern@gmx.com>
Cc:     linux-block@vger.kernel.org, axboe@kernel.dk,
        linux-bcache@vger.kernel.org
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From:   Coly Li <colyli@suse.de>
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Date:   Fri, 26 Feb 2021 17:54:08 +0800
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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.

Coly Li