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    SubjectRe: open(O_DIRECT) on a tmpfs?
    On 04/01/07, Hua Zhong <> wrote:
    > > 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.

    I'd agree. O_DIRECT means data will go direct to backing store, so if
    RAM *is* the backing store as in the tmpfs case, then I see why
    O_DIRECT should fail for it...

    I often use tmpfs when I want to test new setups - it's easy to get
    rid of again and it's fast during testing. Why shouldn't I be able to
    test apps that use O_DIRECT this way?

    Jesper Juhl <>
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      Last update: 2007-01-05 16:01    [W:0.038 / U:3.640 seconds]
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