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

[Christoph Hellwig]

On Mon, Oct 03, 2022 at 11:09:06AM +0300, Sagi Grimberg wrote:
>> make up the multipath device. Only the low-level driver can do that right now,
>> so perhaps either call into the driver to get all the block_device parts, or
>> the gendisk needs to maintain a list of those parts itself.
>
> I definitely don't think we want to propagate the device relationship to
> blk-mq. But a callback to the driver also seems very niche to nvme 
> multipath and is also kinda messy to combine calculations like
> iops/bw/latency accurately which depends on the submission distribution
> to the bottom devices which we would need to track now.
>
> I'm leaning towards just moving forward with this, take the relatively
> small hit, and if people absolutely care about the extra latency, then
> they can disable it altogether (upper and/or bottom devices).

So looking at the patches I'm really not a big fan of the extra
accounting calls, and especially the start_time field in the
nvme_request and even more so the special start/end calls in all
the transport drivers.

the stats sysfs attributes already have the entirely separate
blk-mq vs bio based code pathes.  So I think having a block_device
operation that replaces part_stat_read_all which allows nvme to
iterate over all pathes and collect the numbers would seem
a lot nicer.  There might be some caveats like having to stash
away the numbers for disappearing paths, though.