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Subject: Re: [PATCH] MD: make bio mergeable
To: Ming Lei <ming.lei@canonical.com>, Shaohua Li <shli@fb.com>
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From: Jens Axboe <axboe@kernel.dk>
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Date: Tue, 26 Apr 2016 08:21:21 -0600
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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.

-- 
Jens Axboe