[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.
> 
> It does add up, though, and some environments disable stats to skip the
> overhead. At a minimum, you need to add a check for blk_queue_io_stat() before
> assuming you need to account for stats.
> 
> Instead of duplicating the accounting, could you just have the stats file report
> the sum of its hidden devices?

Interesting...

How do you suggest we do that? .collect_stats() callout in fops?