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    SubjectRE: open(O_DIRECT) on a tmpfs?
    Date
    > I see that as a good argument _not_ to allow O_DIRECT on 
    > tmpfs, which inevitably impacts cache, even if O_DIRECT were
    > requested.
    >
    > But I'd also expect any app requesting O_DIRECT in that way,
    > as a caring citizen, to fall back to going without O_DIRECT
    > when it's not supported.

    According to "man 2 open" on my system:

    O_DIRECT
    Try to minimize cache effects of the I/O to and from this file.
    In general this will degrade performance, but it is useful in
    special situations, such as when applications do their own
    caching. File I/O is done directly to/from user space buffers.
    The I/O is synchronous, i.e., at the completion of the read(2)
    or write(2) system call, data is guaranteed to have been trans-
    ferred. Under Linux 2.4 transfer sizes, and the alignment of
    user buffer and file offset must all be multiples of the logi-
    cal block size of the file system. Under Linux 2.6 alignment to
    512-byte boundaries suffices.
    A semantically similar interface for block devices is described
    in raw(8).

    This says nothing about (probably disk based) persistent backing store. I don't see why tmpfs has to conflict with it.

    So I'd argue that it makes more sense to support O_DIRECT on tmpfs as the memory IS the backing store.

    And EINVAL isn't even a very specific error.

    Hua

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