Date:	Mon, 29 Jun 2009 12:18:41 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Neil Brown <neilb@...e.de>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alasdair G Kergon <agk@...hat.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: [dm-devel] REQUEST for new 'topology' metrics to be moved out
	of the 'queue' sysfs directory.

On Sat, Jun 27 2009, Neil Brown wrote:
> > There's no such thing as first or second class block devices. The fact
> > that drivers using ->make_request_fn directly do not utilize the full
> > scope of the queue isn't a very interesting fact, imho.
> 
>  Your phrase "drivers using ->make_request_fn directly" seems to
>  suggest you are looking at things very differently to me.
> 
>  From my perspective, all drivers use ->make_request_fn equally.
>  Some set it to "__make_request", some to "md_make_request", others to
>  "dm_request" or "loop_make_request" etc.

Neil, will you please stop these silly games. Stop trying to invent
differences based on interpretations of what you read into my replies.

>  Each of these different drivers need some private storage.
>  __make_request uses struct request_queue
>  md_make_request uses struct mddev_s
>  dm_request uses struct mapped_device
>  loop_make_request uses struct loop_device
>   etc
> 
>  These structures are all attached to gendisk one way or another.
> 
>  Of these examples, the first three have an extra level.  They are
>  intermediaries or "midlayers" for multiple drivers and perform some
>  processing before passing the request down.
>  __make_request provides this for ide and scsi (etc) via ->request_fn and
>     ->queuedata in struct request_queue (and other fields).
>  md_make_request provides this for raid1 and raid5 (etc) via
>     ->pers->make_request and  ->private is struct mddev_s (and other
>     fields). 
>  dm_request provides this for crypt and multipath (etc) via
>     ->map->targets[]->type->map and ->map->targets[]->private (and
>     other fields).

Nothing - I repeat nothing - stops md/dm from removing that layer. It's
a layer they imposed themselves based on the design they chose to
implement internally. It has NOTHING to do with how the block layer is
designed. If md raid1 assigned raid1_dev (or whatever raid1 uses a its
device identifier structure) to ->queuedata, and had an mddev_s in its
raid1 structure, that would be a perfectly viable design as well.

Loop does that. md/dm have their own internal layering, if anything is a
"midlayer" (to keep to the apparent theme of design patterns), it's the
code md and dm bits.

>  Looked at from this perspective, the fact that some drivers 'do not
>  utilise the full scope of the queue' certainly isn't the interesting
>  point.  The interesting point is that they have to use parts of the
>  queue at all.
> 
>  And from this perspective, __make_request is a class above everything
>  else.  __make_request gets a dedicate field in gendisk (->queue) and
>  every driver has to provide a queue.  Other (lower class) drivers get
>  to share gendisk->private_date and/or gendisk->queue->queuedata.

That's just utter nonsense.

-- 
Jens Axboe

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