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
Don't love it either.
and even more so the special start/end calls in all
the transport drivers.
The end is centralized and the start part has not sprinkled to
the drivers. I don't think its bad.
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.
You think this is better? really? I don't agree with you, I think its
better to pay a small cost than doing this very specialized thing that
will only ever be used for nvme-mpath.
