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    SubjectRe: clearing filesystem cache for I/O benchmarks
    Benjamin Rutt wrote:

    >Hans Reiser <reiser@namesys.com> writes:
    >
    >
    >
    >>when benchmarking, please be careful that you don't end up
    >>benchmarking umount/mount, or sync, or..... it can be remarkably hard
    >>to avoid such mistakes.....
    >>
    >>
    >
    >I agree, I've made some blunders like that in the past. However for
    >write tests, we are including fsync() time, once, at the end of a file
    >write, since I feel it's unfair to trim that time.
    >
    fsync performance gives you different performance. Better to write more
    stuff to flush the cache.

    > Not including
    >fsync() time would only test the ability of the various parts of the
    >I/O systems to do write buffering. It's easy to do lots of write
    >buffering, if you buy enough memory. Forcing the disks to write is
    >the only fair way to compare writes between I/O systems.
    >
    >
    It isn't fair. fsync is a different code path, and may be less
    efficient. Or more, depending on the fs. reiser4 is currently not well
    optimized for fsync, maybe next year I will change that but not this
    week....

    Benchmarking well is hard.....

    Hans
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