From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751619AbZH0LEb (ORCPT ); Thu, 27 Aug 2009 07:04:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751202AbZH0LEa (ORCPT ); Thu, 27 Aug 2009 07:04:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:33218 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750768AbZH0LEa (ORCPT ); Thu, 27 Aug 2009 07:04:30 -0400 Message-ID: <4A966833.2090404@redhat.com> Date: Thu, 27 Aug 2009 14:04:19 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.1) Gecko/20090814 Fedora/3.0-2.6.b3.fc11 Lightning/1.0pre Thunderbird/3.0b3 MIME-Version: 1.0 To: Rusty Russell CC: Christoph Hellwig , borntraeger@de.ibm.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default References: <20090820205616.GA5503@lst.de> <200908262136.46570.rusty@rustcorp.com.au> <4A952A5D.5040606@redhat.com> <200908272013.50839.rusty@rustcorp.com.au> In-Reply-To: <200908272013.50839.rusty@rustcorp.com.au> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/27/2009 01:43 PM, Rusty Russell wrote: > >>> Are you claiming qcow2 is unusual? I can believe snapshot is less common, >>> though I use it all the time. >>> >>> You'd normally have to add a feature for something like this. I don't >>> think this is different. >>> >> Why do we need to add a feature for this? >> > Because cache=writeback should *not* lie to the guest? > > No, it should. There are two possible semantics to cache=writeback: - simulate a drive with a huge write cache; use fsync() to implement barriers - tell the host that we aren't interested in data integrity, lie to the guest to get best performance The first semantic is not very useful; guests don't expect huge write caches so you can't be sure of your integrity guarantees, and it's slower than cache=none due to double caching and extra copies. The second semantic is not useful for production, but is very useful for testing out things where you aren't worries about host crashes and you're usually rebooting the guest very often (you can't rely on guest caches, so you want the host to cache). -- error compiling committee.c: too many arguments to function