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    SubjectRe: Transparent compression in the FS
    On Thu, Oct 16, 2003 at 04:04:48PM -0700, jw schultz wrote:
    > 2. External collision: The smaller hash size to unit count
    > ratio the greater liklihood of false positives. To put
    > it in the extreme: if you have 10,000 blocks you are
    > almost guaranteed to get false positives on a 16 bit
    > hash. This is similar to filling the namespace. It is
    > external collision that was killing rsync on iso images.

    What about making one bit changes in the block at a random location (the
    same bit on both sides), and comparing the hashes again.

    This would work for something that is trying to save bandwidth over a
    network link, but wouldn't save anything in a Compare-by-hash File System
    (why hash again when you need to read the block anyway?).

    One thought for rsync would be to have a 4 step process (the first two are
    how it operates now according to another sub-thread):

    1) Check the block with a weak 16bit hash

    2) Check the block with a stronger 32bit hash (if step 1 had a collision)

    3) Check the block with SHA1 (if step 2 had a collision)

    4) Make a one bit change at a random location in the block, but at the same
    place on both ends of the transfer and repeat steps 1 to 3

    It's arguable how much of an optimization the 16bit hash is, but it's there
    and maybe it's a win, and maybe not. It's also arguable just how much
    bandwidth you're going to save with the extra steps 3, and the recursive 4.
    Maybe 4 should be just a recheck with SHA1, and the recursive check isn't
    needed, but I haven't done any hash benchmarks.
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