Re: open(O_DIRECT) on a tmpfs?

["Jesper Juhl" <jesper.juhl@gmail.com>]

On 04/01/07, Hua Zhong <hzhong@gmail.com> 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 <jesper.juhl@gmail.com>
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