Messages in this thread |  | | | Date | Fri, 5 Jan 2007 15:58:38 +0100 | | From | "Jesper Juhl" <> | | Subject | Re: open(O_DIRECT) on a tmpfs? |
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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?
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