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SubjectRE: open(O_DIRECT) on a tmpfs?
> 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:

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.


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