From: Avi Kivity Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default Date: Wed, 26 Aug 2009 15:28:13 +0300 Message-ID: <4A952A5D.5040606@redhat.com> References: <20090820205616.GA5503@lst.de> <200908252341.38356.rusty@rustcorp.com.au> <20090825141608.GA32658@lst.de> <200908262136.46570.rusty@rustcorp.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Christoph Hellwig , borntraeger@de.ibm.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org To: Rusty Russell Return-path: Received: from mx1.redhat.com ([209.132.183.28]:51218 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932886AbZHZM2q (ORCPT ); Wed, 26 Aug 2009 08:28:46 -0400 In-Reply-To: <200908262136.46570.rusty@rustcorp.com.au> Sender: kvm-owner@vger.kernel.org List-ID: On 08/26/2009 03:06 PM, Rusty Russell wrote: > On Tue, 25 Aug 2009 11:46:08 pm Christoph Hellwig wrote: > >> On Tue, Aug 25, 2009 at 11:41:37PM +0930, Rusty Russell wrote: >> >>> On Fri, 21 Aug 2009 06:26:16 am Christoph Hellwig wrote: >>> >>>> Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which >>>> means it does not allow filesystems to use barriers. But the typical use >>>> case for virtio-blk is to use a backed that uses synchronous I/O >>>> >>> Really? Does qemu open with O_SYNC? >>> >>> I'm definitely no block expert, but this seems strange... >>> Rusty. >>> >> Qemu can open it various ways, but the only one that is fully safe >> is O_SYNC (cache=writethrough). >> > (Rusty goes away and reads the qemu man page). > > By default, if no explicit caching is specified for a qcow2 disk image, > cache=writeback will be used. > It's now switched to writethrough. In any case, cache=writeback means "lie to the guest, we don't care about integrity". > 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? -- error compiling committee.c: too many arguments to function