Re: [PATCH] MD: make bio mergeable

[Jens Axboe]

On 04/26/2016 03:56 AM, Ming Lei wrote:
> On Tue, Apr 26, 2016 at 7:52 AM, Shaohua Li <shli@xxxxxx> 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