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    SubjectRe: O_DIRECT question
    Linus Torvalds wrote:
    > On Wed, 10 Jan 2007, Linus Torvalds wrote:
    >>So don't use O_DIRECT. Use things like madvise() and posix_fadvise()
    > Side note: the only reason O_DIRECT exists is because database people are
    > too used to it, because other OS's haven't had enough taste to tell them
    > to do it right, so they've historically hacked their OS to get out of the
    > way.
    > As a result, our madvise and/or posix_fadvise interfaces may not be all
    > that strong, because people sadly don't use them that much. It's a sad
    > example of a totally broken interface (O_DIRECT) resulting in better
    > interfaces not getting used, and then not getting as much development
    > effort put into them.
    > So O_DIRECT not only is a total disaster from a design standpoint (just
    > look at all the crap it results in), it also indirectly has hurt better
    > interfaces. For example, POSIX_FADV_NOREUSE (which _could_ be a useful and
    > clean interface to make sure we don't pollute memory unnecessarily with
    > cached pages after they are all done) ends up being a no-op ;/
    > Sad. And it's one of those self-fulfilling prophecies. Still, I hope some
    > day we can just rip the damn disaster out.

    Speaking of which, why did we obsolete raw devices? And/or why not just
    go with a minimal O_DIRECT on block device support? Not a rhetorical
    question -- I wasn't involved in the discussions when they happened, so
    I would be interested.

    O_DIRECT is still crazily racy versus pagecache operations. Chris Mason's
    recent patches to attempt to fix it, while actually doing quite a fine
    job, are very intrusive and complex for such a sad corner case.

    SUSE Labs, Novell Inc.
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