On 2011-04-11 14:05, NeilBrown wrote: > 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. OK fair enough, I'll add your callback patch. -- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
