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    SubjectRe: [PATCH 05/10] block: remove per-queue plugging
    On 2011-04-11 14:05, NeilBrown wrote:
    > On Mon, 11 Apr 2011 13:37:20 +0200 Jens Axboe <jaxboe@fusionio.com> wrote:
    >
    >> On 2011-04-11 13:26, NeilBrown wrote:
    >>> 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.
    >>
    >> 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



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