From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([140.186.70.92]:42295) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QArzx-0002Mu-W6 for qemu-devel@nongnu.org; Fri, 15 Apr 2011 19:00:23 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1QArzw-0006Ti-Vj for qemu-devel@nongnu.org; Fri, 15 Apr 2011 19:00:21 -0400 Received: from e38.co.us.ibm.com ([32.97.110.159]:48652) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1QArzw-0006Td-KS for qemu-devel@nongnu.org; Fri, 15 Apr 2011 19:00:20 -0400 Received: from d03relay03.boulder.ibm.com (d03relay03.boulder.ibm.com [9.17.195.228]) by e38.co.us.ibm.com (8.14.4/8.13.1) with ESMTP id p3FMifaf023866 for ; Fri, 15 Apr 2011 16:44:41 -0600 Received: from d03av06.boulder.ibm.com (d03av06.boulder.ibm.com [9.17.195.245]) by d03relay03.boulder.ibm.com (8.13.8/8.13.8/NCO v10.0) with ESMTP id p3FN0Ig8118500 for ; Fri, 15 Apr 2011 17:00:18 -0600 Received: from d03av06.boulder.ibm.com (loopback [127.0.0.1]) by d03av06.boulder.ibm.com (8.14.4/8.13.1/NCO v10.0 AVout) with ESMTP id p3FN5HdR002634 for ; Fri, 15 Apr 2011 17:05:17 -0600 Message-ID: <4DA8CE00.3090907@us.ibm.com> Date: Fri, 15 Apr 2011 18:00:16 -0500 From: Anthony Liguori MIME-Version: 1.0 References: <1302874855-14736-1-git-send-email-stefanha@linux.vnet.ibm.com> <20110415150513.GA29619@lst.de> <20110415153448.GA30116@lst.de> <1302884634.32391.3.camel@badari-desktop> <20110415172909.GB303@lst.de> <4DA8C4F0.4080507@us.ibm.com> In-Reply-To: <4DA8C4F0.4080507@us.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on Linux NFS List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: pbadari@linux.vnet.ibm.com Cc: Kevin Wolf , Stefan Hajnoczi , Stefan Hajnoczi , qemu-devel@nongnu.org, Khoa Huynh , Christoph Hellwig On 04/15/2011 05:21 PM, pbadari@linux.vnet.ibm.com wrote: > On 4/15/2011 10:29 AM, Christoph Hellwig wrote: >> On Fri, Apr 15, 2011 at 09:23:54AM -0700, Badari Pulavarty wrote: >>> True. That brings up a different question - whether we are doing >>> enough testing on mainline QEMU :( >> It seems you're clearly not doing enough testing on any qemu. Even >> the RHEL6 qemu has had preadv/pwritev since the first beta. > > Christoph, > > When you say "you're" - you really meant RH right ? RH should have > caught this in their > regression year ago as part of their first beta. Correct ? > > Unfortunately, you are picking on person who spent time find & > analyzing the regression, > narrowing the problem area and suggesting approaches to address the > issue :( This is a pretty silly discussion to be having. The facts are: 1) NFS sucks with preadv/pwritev and O_DIRECT -- is anyone really surprised? 2) We could work around this in QEMU by doing something ugly 3) We have no way to detect when we no longer need a work around which makes (2) really unappealing. 4) That leaves us with: a) waiting for NFS to get fixed properly and just living with worse performance on older kernels b) having a user-tunable switch to enable bouncing I really dislike the idea of (b) because we're stuck with it forever and it's yet another switch for people to mistakenly depend on. I'm still waiting to see performance data without O_DIRECT. I suspect that using cache=writethrough will make most of this problem go away in which case, we can just recommend that as a work around until NFS is properly fixed. Regards, Anthony Liguori