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    SubjectRe: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co
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    Jens Axboe <axboe@kernel.dk> writes:

    > On 2010-06-21 11:48, Christoph Hellwig wrote:
    >> Now how do we use these flags in the block layer?
    >>
    >> - REQ_META
    >>
    >> The only place where we ever use this flag is inside the
    >> cfq scheduler. In cfq_choose_req we use it to give a meta
    >> request priority over one that doesn't have it. But before
    >> that we already do the same preference check with rw_is_sync,
    >> which evaluates to true for requests with that are either
    >> reads or have REQ_SYNC set. So for reads the REQ_META flag
    >> here effectively is a no-op, and for writes it gives less
    >> priority than REQ_SYNC.
    >> In addition to that we use it to account for pending metadata
    >> requests in cfq_rq_enqueued/cfq_remove_request which gets
    >> checked in cfq_should_preempt to give priority to a meta
    >> request if the other queue doesn't have any pending meta
    >> requests. But again this priority comes after a similar
    >> check for sync requests that checks if the other queue has
    >> been marked to have sync requests pending.
    >
    > It's also annotation for blktrace, so you can tell which parts of the IO
    > is meta data etc. The scheduler impact is questionable, I doubt it makes
    > a whole lot of difference.

    Really? Even after I showed the performance impact of setting that bit
    for journal I/O?

    http://lkml.org/lkml/2010/4/1/344

    Cheers,
    Jeff


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