Re: [PATCH 05/10] block: remove per-queue plugging

[NeilBrown <neilb@suse.de>]

On Mon, 11 Apr 2011 13:04:26 +0200 Jens Axboe <jaxboe@fusionio.com> wrote:

> > 
> > I'm sure one of us is missing something (probably both) but I'm not
> > sure what.
> > 
> > The callback is central.
> > 
> > It is simply to use plugging in md.
> > Just like blk-core, md will notice that a blk_plug is active and will put
> > requests aside.  I then need something to call in to md when blk_finish_plug
> 
> But this is done in __make_request(), so md devices should not be
> affected at all. This is the part of your explanation that I do not
> connect with the code.
> 
> If md itself is putting things on the plug list, why is it doing that?

Yes.  Exactly.  md itself want to put things aside on some list.
e.g. in RAID1 when using a write-intent bitmap I want to gather as many write
requests as possible so I can update the bits for all of them at once.
So when a plug is in effect I just queue the bios somewhere and record the
bits that need to be set.
Then when the unplug happens I write out the bitmap updates in a single write
and when that completes, I write out the data (to all devices).

Also in RAID5 it is good if I can wait for lots of write request to arrive
before committing any of them to increase the possibility of getting a
full-stripe write.

Previously I used ->unplug_fn to release the queued requests.  Now that has
gone I need a different way to register a callback when an unplug happens.

> 
> > is called so that put-aside requests can be released.
> > As md can be built as a module, that call must be a call-back of some sort.
> > blk-core doesn't need to register blk_plug_flush because that is never in a
> > module, so it can be called directly.  But the md equivalent could be in a
> > module, so I need to be able to register a call back.
> > 
> > Does that help? 
> 
> Not really. Is the problem that _you_ would like to stash things aside,
> not the fact that __make_request() puts things on a task plug list?
> 

Yes, exactly.  I (in md) want to stash things aside.

(I don't actually put the stashed things on the blk_plug, though it might
make sense to do that later in some cases - I'm not sure.  Currently I stash
things in my own internal lists and just need a call back to say "ok, flush
those lists now").

Thanks,
NeilBrown