On 04/01/07, Hua Zhong <[email protected]> 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 <[email protected]>
Don't top-post http://www.catb.org/~esr/jargon/html/T/top-post.html
Plain text mails only, please http://www.expita.com/nomime.html
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