[PATCH rfc] nvme: support io stats on the mpath device

[Sagi Grimberg]

>>> 3. Do you have some performance numbers (we're touching the fast path here) ?
>>
>> This is pretty light-weight, accounting is per-cpu and only wrapped by
>> preemption disable. This is a very small price to pay for what we gain.
> 
> Is it? Enabling IO stats for normal devices has a very noticeable impact
> on performance at the higher end of the scale.

Interesting, I didn't think this would be that noticeable. How much
would you quantify the impact in terms of %?

I don't have any insight on this for blk-mq, probably because I've never
seen any user turn IO stats off (or at least don't remember).

My (very limited) testing did not show any noticeable differences for
nvme-loop. All I'm saying that we need to have IO stats for the mpath
device node. If there is a clever way to collect this from the hidden
devices just for nvme, great, but we need to expose these stats.

> So much so that I've contemplated how we can make this less expensive than it currently is.

Then nvme-mpath would benefit from that as well.