On Mon, 11 Apr 2011 13:37:20 +0200 Jens Axboe <jaxboe@xxxxxxxxxxxx> wrote: > On 2011-04-11 13:26, NeilBrown wrote: > > 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. > > Ah, so this is what I was hinting at. But why use the task->plug for > that? Seems a bit counter intuitive. Why can't you just store these > internally? > > > > >> > >>> 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"). > > So we are making some progress... The thing I then don't understand is > why you want to make it associated with the plug? Seems you don't have > any scheduling restrictions, and in which case just storing them in md > seems like a much better option. > Yes. But I need to know when to release the requests that I have stored. I need to know when ->write_pages or ->read_pages or whatever has finished submitting a pile of pages so that I can start processing the request that I have put aside. So I need a callback from blk_finish_plug. (and I also need to know if a thread that was plugging schedules for the same reason that you do). NeilBrown -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
