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    SubjectRe: dd PATCH: add conv=direct
    On Wed, Apr 07, 2004 at 01:46:31PM -0700, Paul Eggert wrote:
    > Andrew Morton <> writes:
    > > In 2.6 we do the check at open() and fcntl() time. In 2.4 we don't
    > > fail until the actual I/O attempt.
    > This raises the issue of what "dd conv=direct" should do in 2.4
    > kernels. I propose that it should report an error and exit, when the
    > write fails, since conv=direct can't be implemented. The basic idea
    > is that on systems that lack direct I/O, conv=direct should fail.

    I agree, this is the appropriate behavior. When "dd conv=direct" is
    used on a fd that cannot support direct I/O, and the kernel doesn't tell
    us until it's too late, erroring on the I/O is the only sane action.

    Fortuitously, that's what happens with the patches I posted.

    (Note that it's a function of the kernel and the underlying FS whether
    direct I/O is going to work. So some kind of heuristic on kernel
    version is a bad idea.)

    > Another issue with this patch: in Solaris, direct I/O is done by
    > invoking directio(DIRECTIO_ON); see
    > <>.
    > Is Solaris direct I/O a direct analog to Linux direct I/O, or are
    > there subtle differences in semantics that should be made visible to
    > the users of GNU "dd"?

    The Solaris semantics are more sane than the Linux ones, to be sure --
    it always does the I/O, falling back to buffered I/O if conditions are
    not right for direct.

    I don't believe I have a test system for the Solaris case (when was
    directio(3C) added?) so someone else will have to add that support.

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