Re: IO request merging

[Jens Axboe]

On 10/16/2014 06:27 AM, Jan Kara wrote:
>   Hello,
> 
>   one of our customers was complaining that elv_attempt_insert_merge()
> merges two requests (via blk_attempt_req_merge()) without asking IO
> scheduler for permission (->elevator_allow_merge_fn() callback). Now for
> them this is a problem because of their custom IO scheduler but looking
> into the code this can result in somewhat suboptimal behavior for CFQ as
> well (merging two requests from different IO contexts, possibly merging
> sync & async request). What do others think about this?
> 
> Regarding possible fix, we cannot really call ->elevator_allow_merge_fn()
> because that assumes it is called from a context of a process submitting the
> passed bio. So we would need to create a separate allow merge callback for
> this.

It would need a new (rq to rq) merge hook, if they have a custom IO
scheduler, they should submit a change to allow that kind of behaviour.
Outside of potentially mixing sync and async IO (which seems like
something that should rarely/never happen), not sure I see a lot of
downsides. And that case could be explicitly checked in attempt_merge()
or blk_attempt_req_merge() without having to define a new hook to catch
that specific case. For the hook, cfq would lookup the io contexts and
compare, and basically disallow any merge that crosses a cfq io context
boundary. But given that I would only expect these types of merges to
happen very rarely, the sync vs async check would be good enough for me.

-- 
Jens Axboe

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