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    SubjectRe: [PATCH 1/3] block_write_full_page: Use synchronous writes for WBC_SYNC_ALL writebacks
    On Mon, 6 Apr 2009 23:21:41 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

    > I mean, let's graph it:
    >
    > WRITE_SYNC -> WRITE_SYNC_PLUG -> BIO_RW_SYNCIO -> bio_sync() -> REQ_RW_SYNC -> rw_is_sync() -> does something mysterious in get_request()
    > -> rq_is_sync() -> does something mysterious in IO schedulers
    > -> BIO_RW_NOIDLE -> bio_noidle() -> REQ_NOIDLE -> rq_noidle() -> does something mysterious in cfq-iosched only
    > -> BIO_RW_UNPLUG -> bio_unplug() -> REQ_UNPLUG -> OK, the cognoscenti know what this is supposed to do, but it is unused!

    whoop, I found a use of bio_unplug() in __make_request().

    So it appears that the intent of your patch is to cause an unplug after
    submission of each WB_SYNC_ALL block?

    But what about all the other stuff which WRITE_SYNC might or might not
    do? What does WRITE_SYNC _actually_ do, and what are the actual
    effects of this change??

    And what effect will this large stream of unplugs have upon merging?


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