From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [Qemu-devel] qemu and qemu.git -> Migration + disk stress introduces qcow2 corruptions Date: Tue, 15 Nov 2011 15:28:02 +0200 Message-ID: <4EC268E2.1020407@redhat.com> References: <4EBAAA68.10801@redhat.com> <4EBAACAF.4080407@codemonkey.ws> <4EBAB236.2060409@redhat.com> <4EBAB9FA.3070601@codemonkey.ws> <4EBB919B.7040605@redhat.com> <4EBC1792.3030004@codemonkey.ws> <4EBC4260.1090405@codemonkey.ws> <4EBCF5DA.1000605@redhat.com> <4EBE499E.4030100@redhat.com> <4EC0E638.8030908@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Anthony Liguori , Lucas Meneghel Rodrigues , KVM mailing list , "Michael S. Tsirkin" , Marcelo Tosatti , QEMU devel , Juan Jose Quintela Carreira , "Daniel P. Berrange" , "libvir-list@redhat.com" To: Kevin Wolf Return-path: Received: from mx1.redhat.com ([209.132.183.28]:26680 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754021Ab1KON2O (ORCPT ); Tue, 15 Nov 2011 08:28:14 -0500 In-Reply-To: <4EC0E638.8030908@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 11/14/2011 11:58 AM, Kevin Wolf wrote: > Am 12.11.2011 11:25, schrieb Avi Kivity: > > On 11/11/2011 12:15 PM, Kevin Wolf wrote: > >> Am 10.11.2011 22:30, schrieb Anthony Liguori: > >>> Live migration with qcow2 or any other image format is just not going to work > >>> right now even with proper clustered storage. I think doing a block level flush > >>> cache interface and letting block devices decide how to do it is the best approach. > >> > >> I would really prefer reusing the existing open/close code. It means > >> less (duplicated) code, is existing code that is well tested and doesn't > >> make migration much of a special case. > >> > >> If you want to avoid reopening the file on the OS level, we can reopen > >> only the topmost layer (i.e. the format, but not the protocol) for now > >> and in 1.1 we can use bdrv_reopen(). > > > > Intuitively I dislike _reopen style interfaces. If the second open > > yields different results from the first, does it invalidate any > > computations in between? > > Not sure what results and what computation you mean, Result = open succeeded. Computation = anything that derives from the image, like size, or reading some stuff to guess CHS or something. > but let me clarify > a bit about bdrv_reopen: > > The main purpose of bdrv_reopen() is to change flags, for example toggle > O_SYNC during runtime in order to allow the guest to toggle WCE. This > doesn't necessarily mean a close()/open() sequence if there are other > means to change the flags, like fcntl() (or even using other protocols > than files). > > The idea here was to extend this to invalidate all caches if some > specific flag is set. As you don't change any other flag, this will > usually not be a reopen on a lower level. > > If we need to use open() though, and it fails (this is really the only > "different" result that comes to mind) (yes) > then bdrv_reopen() would fail and > the old fd would stay in use. Migration would have to fail, but I don't > think this case is ever needed for reopening after migration. Okay. > > > What's wrong with just delaying the open? > > Nothing, except that with today's code it's harder to do. > This has never stopped us (though it may delay us). -- error compiling committee.c: too many arguments to function