linux-kernel - Re: open(O_DIRECT) on a tmpfs?

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Date:	Fri, 5 Jan 2007 15:59:41 +0100
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Hua Zhong" <hzhong@...il.com>
Cc:	"Hugh Dickins" <hugh@...itas.com>,
	"Bill Davidsen" <davidsen@....com>,
	Linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: open(O_DIRECT) on a tmpfs?

On 05/01/07, Jesper Juhl <jesper.juhl@...il.com> wrote:
> On 04/01/07, Hua Zhong <hzhong@...il.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...
>
Whoops, that should of course have read " then I *DON'T* see why
O_DIRECT should fail" .

-- 
Jesper Juhl <jesper.juhl@...il.com>
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