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SubjectRe: [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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