Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1161130AbXAEO7n (ORCPT ); Fri, 5 Jan 2007 09:59:43 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1161128AbXAEO7n (ORCPT ); Fri, 5 Jan 2007 09:59:43 -0500 Received: from wx-out-0506.google.com ([66.249.82.231]:48739 "EHLO wx-out-0506.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1161130AbXAEO7m (ORCPT ); Fri, 5 Jan 2007 09:59:42 -0500 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=beta; d=gmail.com; h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=cRAE5Q0KgEoB58dQ7+ZiHZB/viwRgOJ8u3G5lKPZUAYfR9N+WLxQ1PNWZo4Lwi3xZbnNkoctKs5/rRHavK2WEV593Ib0/gOKH3QjtU3lydY8IE3/9PjgJhEwvnwe/ULIx3+sgsOiJazu2U+2Uq0KZvnxbGz4kS3jyWZP+WwFAq4= Message-ID: <9a8748490701050659r3715aa48j883c26201005d26c@mail.gmail.com> Date: Fri, 5 Jan 2007 15:59:41 +0100 From: "Jesper Juhl" To: "Hua Zhong" Subject: Re: open(O_DIRECT) on a tmpfs? Cc: "Hugh Dickins" , "Bill Davidsen" , Linux-kernel In-Reply-To: <9a8748490701050658n39f82791yb3d2e3c80f237a7e@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <003f01c7302f$e72164b0$0200a8c0@nuitysystems.com> <9a8748490701050658n39f82791yb3d2e3c80f237a7e@mail.gmail.com> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2323 Lines: 47 On 05/01/07, Jesper Juhl wrote: > On 04/01/07, Hua Zhong 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 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/