On 2011-04-18 11:19, hch@infradead.org wrote:

Btw, I really start to wonder if the request level is the right place
to do this on-stack plugging.  Wouldn't it be better to just plug
bios in the on-stack queue?  That way we could also stop doing the
special case merging when adding to the plug list, and leave all the
merging / I/O schedule logic in the __make_request path.  Probably
not .39 material, but worth a prototype?

Also what this dicussion brought up is that the block layer data
structures are highly confusing.  Using a small subset of the
request_queue also for make_request based driver just doesn't make
sense.  It seems like we should try to migrate the required state
to struct gendisk, and submit I/O through a block_device_ops.submit
method, leaving the request_queue as an internal abstraction for
the request based drivers.

Partially agree, I've never really liked the two methods we have where
the light light version was originally meant for stacked devices but
gets used elsewhere now too. It also causes IO scheduling problems, and
then you get things like request based dm to work around that.

But the idea is really to move more towards private queueing more
localized, the multiqueue setup will really apply well there too. I'm
trying to flesh out the design of that, ideally it would be nice to
unify the different bits we have now.

But agree on pulling the stacked bits into some lower part, like the
gendisk. It would clean that up nicely.


-- 
Jens Axboe