On Mon, 11 Apr 2011 13:04:26 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
