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    SubjectRe: O_DIRECT! or O_DIRECT?
    Hi,

    On Wed, Jul 04, 2001 at 06:27:13PM +0000, Miquel van Smoorenburg wrote:
    > In article <20010704185230.F28793@redhat.com>,
    > Stephen C. Tweedie <sct@redhat.com> wrote:
    > >For these reasons, buffered IO is often faster than O_DIRECT for pure
    > >sequential access. The downside it its greater CPU cost and the fact
    > >that it pollutes the cache (which, in turn, causes even _more_ CPU
    > >overhead when the VM is forced to start reclaiming old cache data to
    > >make room for new blocks.)
    >
    > Any chance of something like O_SEQUENTIAL (like madvise(MADV_SEQUENTIAL))

    What for? The kernel already optimises readahead and writebehind for
    sequential files.

    If you want to provide specific extra hints to the kernel, then things
    like O_UNCACHE might be more appropriate to instruct the kernel to
    explicitly remove the cached page after IO completes (to avoid the VM
    overhead of maintaining useless cache). That would provide a definite
    improvement over normal IO for large multimedia-style files or for
    huge copies. But what part of the normal handling of sequential files
    would O_SEQUENTIAL change? Good handling of sequential files should
    be the default, not an explicitly-requested feature.

    Cheers,
    Stephen
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