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Date: Tue, 26 Apr 2016 23:17:57 +0800
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Subject: Re: [PATCH] MD: make bio mergeable
From: Ming Lei <ming.lei@canonical.com>
To: Jens Axboe <axboe@kernel.dk>
Cc: Shaohua Li <shli@fb.com>, linux-block@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "open list:SOFTWARE RAID (Multiple Disks) SUPPORT" 
	<linux-raid@vger.kernel.org>,
        Ju Hyung Park <qkrwngud825@gmail.com>,
        FB Kernel Team <Kernel-team@fb.com>, "v4.3+" <stable@vger.kernel.org>,
        Jens Axboe <axboe@fb.com>, Neil Brown <neilb@suse.de>
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On Tue, Apr 26, 2016 at 10:21 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 04/26/2016 03:56 AM, Ming Lei wrote:
>>
>> On Tue, Apr 26, 2016 at 7:52 AM, Shaohua Li <shli@fb.com> wrote:
>>>
>>> blk_queue_split marks bio unmergeable, which makes sense for normal bio.
>>> But if dispatching the bio to underlayer disk, the blk_queue_split
>>> checks are invalid, hence it's possible the bio becomes mergeable.
>>
>>
>> If the bio from md is splitted and marked as NOMERGE, it means some
>> queue limits are reached. So looks the raid's queue limit is set as not
>> big enough, could your find which limit causes the splitting and nomerge?
>
>
> raid0 sets a limit of the stripe size for IO. Once the IO has passed md,
> there's no reason why we can't merge for the lower driver. This is
> (potentially) a huge performance issue on trim, since a lot of devices are
> trim ops / sec limited rather than throughput limited.

Just found raid0 maps the chunk sectors into max hw sectors of queue,
and dm uses blk_stack_limits() to set up the limits.

So looks a raid specific issue, then the fix is correct, sorry for the noise.

thanks,
Ming