From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:39790) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1ZmtFv-0002Dg-Fz for qemu-devel@nongnu.org; Thu, 15 Oct 2015 20:52:24 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1ZmtFp-0005cx-Vc for qemu-devel@nongnu.org; Thu, 15 Oct 2015 20:52:22 -0400 References: <1443161858-20533-1-git-send-email-wency@cn.fujitsu.com> <1443161858-20533-9-git-send-email-wency@cn.fujitsu.com> <20151012162714.GC4053@stefanha-thinkpad.redhat.com> <561CCA01.7000001@cn.fujitsu.com> <20151014142751.GD16162@stefanha-thinkpad.redhat.com> <561F0D25.9000409@cn.fujitsu.com> <20151015145558.GA21733@stefanha-thinkpad.redhat.com> From: Wen Congyang Message-ID: <56204A31.8080006@cn.fujitsu.com> Date: Fri, 16 Oct 2015 08:52:01 +0800 MIME-Version: 1.0 In-Reply-To: <20151015145558.GA21733@stefanha-thinkpad.redhat.com> Content-Type: text/plain; charset="windows-1252" Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v10 08/10] Implement new driver for block replication List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Kevin Wolf , Fam Zheng , zhanghailiang , qemu block , Stefan Hajnoczi , Jeff Cody , Jiang Yunhong , Dong Eddie , qemu devel , "Michael R. Hines" , Max Reitz , Gonglei , Paolo Bonzini , Yang Hongyang , "Dr. David Alan Gilbert" On 10/15/2015 10:55 PM, Stefan Hajnoczi wrote: > On Thu, Oct 15, 2015 at 10:19:17AM +0800, Wen Congyang wrote: >> On 10/14/2015 10:27 PM, Stefan Hajnoczi wrote: >>> On Tue, Oct 13, 2015 at 05:08:17PM +0800, Wen Congyang wrote: >>>> On 10/13/2015 12:27 AM, Stefan Hajnoczi wrote: >>>>> On Fri, Sep 25, 2015 at 02:17:36PM +0800, Wen Congyang wrote: >>>>>> + /* start backup job now */ >>>>>> + bdrv_op_unblock(s->hidden_disk, BLOCK_OP_TYPE_BACKUP_TARGET, >>>>>> + s->active_disk->backing_blocker); >>>>>> + bdrv_op_unblock(s->secondary_disk, BLOCK_OP_TYPE_BACKUP_SOURCE, >>>>>> + s->hidden_disk->backing_blocker); >>>>> >>>>> Why is it safe to unblock these operations? >>>>> >>>>> Why do they have to be blocked for non-replication users? >>>> >>>> hidden_disk and secondary disk are opened as backing file, so it is blocked for >>>> non-replication users. >>>> What can I do if I don't unblock it and want to do backup? >>> >>> CCing Jeff Cody, block jobs maintainer >>> >>> You need to explain why it is safe remove this protection. We can't >>> merge code that may be unsafe. >>> >>> I think we can investigate further by asking: when does QEMU code assume >>> the backing file is read-only? >> >> The backing file is opened in read-only mode. I want to reopen it in read-write >> mode here in the next version(So the patch 1 will be dropped) >> >>> >>> I haven't checked but these cases come to mind: >>> >>> Operations that move data between BDS in the backing chain (e.g. commit >>> and stream block jobs) will lose or overwrite data if the backing file >>> is being written to by another coroutine. >>> >>> We need to prevent users from running these operations at the same time. >> >> Yes, but qemu doesn't provide such API. > > This series can't be merged unless it is safe. > > Have you looked at op blockers and thought about how to prevent unsafe > operations? Hmm, only block jobs will write to the backing file? If so, op blockers can prevent unsafe operations. Thanks Wen Congyang > > Stefan > . >