FreeBSD 5.3 I/O Performance / Linux 2.6.10 and dragonfly

Matthew Dillon dillon at
Wed Feb 2 13:59:00 PST 2005

    Urmmm.  how about a bit more information... what are the machine
    configurations?  The disk topology?  The networking?  The graphs are
    almost completely unannotated, it's hard to figure out what the numbers
    actually mean.

    I can figure some things out.  Clearly the BSD write numbers are dropping
    at a block size of 2048 due to vfs.write_behind being set to 1.  Just as
    clearly, Linux is not bothering to write out ANY data, and then able to
    take advantage of the fact that the test file is being destroyed by
    iozone (so it can throw away the data rather then write it out).  This
    skews the numbers to the point where the benchmark doesn't even come close
    to reflecting reality, though I do believe it points to an issue with
    the BSDs ... the write_behind heuristic is completely out of date now 
    and needs to be reworked.

    The read tests are less clear.  iozone runs its read tests just after
    it runs its write tests. so filesystem syncing and write flushing is
    going to have a huge effect on the read numbers.  I suspect that this
    is skewing the results across the spectrum.  In particular, I don't
    see anywhere near the difference in cache-read performance between
    FreeBSD-5 and DragonFly.  But I guess I'll have to load up a few test
    boxes myself and do my own comparisons to figure out what is going on.


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