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    SubjectRe: Servicing of requests
    On Sun, Mar 23 2003, Subodh S wrote:
    > Hi,
    > Whenever I read data of 'x'k size using one read() system call, I find
    > batches of some 'y' no. of make_requests calls followed by the same
    > no. of end_io's. Something like :

    > make_req
    > make_req
    > make_req
    > end_io
    > end_io
    > end_io
    > make_req
    > make_req
    > make_req
    > end_io
    > end_io
    > end_io
    > The output above gives me an idea that 3(hypothetical no.)
    > buffer_heads above form a request. (since 1 make_request corresponds
    > to 1 buffer_head) and maybe since 1 request is serviced at a time I
    > can see 3 make_req's together. Is my understanding right ??
    > But, I have read that sd uses some optimization algorithm to club
    > requests so that the disk seek time is reduced. In which case since
    > all requests are to adjecents sectors it should create a single
    > request of all 'x'k assuming 1 buffer_head is of size 1k.
    > Does this make sense ??

    First of all, please line wrap your emails at 72 chars. Your mail reads

    Second, what is your question? Yes typically buffer_heads can get
    clustered into a request so that contig regions on disk are handed to
    the driver as a single request that may contain X buffer_heads. sd
    doesn't do this on its own, the block layer does it for the driver. And
    it happens for all drivers.

    Jens Axboe

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