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    SubjectRe: [NBD] patch and documentation
    >>The patch also looks harmless enough for applying ;-).
    > Harmless enough, although I'm not sure it really makes that much
    > difference. The max_sectors being set to 255 doesn't, by itself, explain
    > the back and forth 127k, 1k request thing. Typically what you'll see is
    > 127k, 127k, 127k, etc. and then some odd sized request at the end. Or
    > the device gets unplugged anyway at some point and there are odd sized
    > requests scattered throughout...that's especially going to be true if
    > the reads or writes are from an actual disk, rather than /dev/null. I
    > may be just coincidence that setting max_sectors to 256 actually helps.
    > Also, are we sure that all those requests you're seeing are of the same
    > type (all reads, all writes)?

    Well, i guess the cache uses a value of 256 sectors to do read-ahead and
    such. I used dd if=/dev/nbd/0 of=/dev/null bs=X with both X=1 and X=1M.
    Both with the same result. That the 1byte requests join together to
    bigger ones can only be explained with read-aheads strategies.
    Anyway, the result is always the same:

    without patch: 127KB, 1KB, 127KB, 1KB
    with path: 128KB, 128KB, 128KB

    As long as dd doesn't write i'm sure that i didn't see any write
    requests. In addition it is a very regular pattern.
    If it is really the case that the cache reads 256 sectors and the
    default limit is 255, than this would also happen for all other
    block-devices. In addition it would be a good thing to look up if the
    cache takes the max_sectors stuff into accout while determining the
    amout of sectors it reads ahead.

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