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    SubjectRe: [Scst-devel] ISCSI-SCST performance (with also IET and STGT data)
    On Mon, Mar 30, 2009 at 8:33 PM, Vladislav Bolkhovitin <> wrote:
    > Bart Van Assche, on 03/30/2009 10:06 PM wrote:
    >> These are indeed interesting results. There are some aspects of the
    >> test setup I do not understand however:
    >> * All tests have been run with buffered I/O instead of direct I/O
    >> (iflag=direct / oflag=direct). My experience is that the results of
    >> tests with direct I/O are easier to reproduce (less variation between
    >> runs). So I have been wondering why the tests have been run with
    >> buffered I/O instead ?
    > Real applications use buffered I/O, hence it should be used in tests. It
    >  evaluates all the storage stack on both initiator and target as a whole.
    > The results are very reproducible, variation is about 10%.

    Most applications do indeed use buffered I/O. Database software
    however often uses direct I/O. It might be interesting to publish
    performance results for both buffered I/O and direct I/O. A quote from
    the paper "Asynchronous I/O Support in Linux 2.5" by Bhattacharya e.a.
    (Linux Symposium, Ottawa, 2003):

    Direct I/O (raw and O_DIRECT) transfers data between a user buffer and
    a device without copying the data through the kernel’s buffer cache.
    This mechanism can boost performance if the data is unlikely to be
    used again in the short term (during a disk backup, for example), or
    for applications such as large database management systems that
    perform their own caching.

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