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    SubjectRe: [PATCH v1 00/16] ext4: Add metadata checksumming
    >>>>> "Andi" == Andi Kleen <> writes:

    Andi> Doesn't have any performance numbers.

    It's been a while since I read them. I thought they had some compelling
    numbers. Anyway, made a big difference in real life testing here. For
    sustained I/O we're talking an order of magnitude.

    Andi> You need to keep in mind that PCLMULQDQ uses FPU state, so any
    Andi> speedup for the kernel must be large enough to amortize the cost
    Andi> of saving the FPU state.

    Yeah, my test cases were for bulk database I/O, not for writing a
    handful of fs metadata blocks. Plus for the DB tests the CRC was
    generated in userland.

    I seem to recall Joel picking something other than the hw-accelerated
    CRC32C for ocfs2 metadata and that didn't cause any problems.

    That said, I do see a difference between IP checksum and CRC on normal
    FS workloads with DIX enabled here.

    Andi> Typically that only works out for quite large buffers, but kernel
    Andi> buffers are relatively small.


    Martin K. Petersen Oracle Linux Engineering

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