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    SubjectRe: dd PATCH: add conv=direct
    Paul Eggert <eggert@CS.UCLA.EDU> 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,

    And when the read 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 think that's best. It's a bit user-unfriendly, but a silent fallback is

    It might be acceptable to print a warning, then fall back.

    > 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"?

    solaris directio(DIRECTIO_ON) is a hint only. If the system cannot perform
    the uncached zerocopy then it will fall back to buffered IO. Solaris will
    perform direct-IO "when the application's buffer is aligned on a two-byte
    (short) boundary, the offset into the file is on a device sector boundary,
    and the size of the operation is a multiple of device sectors."

    On Linux you set direct-io with open(O_DIRECT) or fcntl(F_SETFL, O_DIRECT).
    If the filesystem doesn't support direct-io we will fail the open/fcntl
    attempt up-front in 2.6 only.

    If O_DIRECT was successfully set and the IO is not correctly aligned Linux
    will fail the relevant I/O attempt. Alignment requirements are:

    2.4: page aligned on-disk and in-memory

    2.6: device sector aligned on-disk and in-memory.

    I'd recomment that dd use getpagesize() alignment on-disk and in-memory.
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