From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([2001:4830:134:3::10]:40031)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1YlE0U-0007Cx-Mr
	for qemu-devel@nongnu.org; Thu, 23 Apr 2015 06:05:19 -0400
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <pbonzini@redhat.com>) id 1YlE0U-000378-13
	for qemu-devel@nongnu.org; Thu, 23 Apr 2015 06:05:18 -0400
Message-ID: <5538C3CC.9030902@redhat.com>
Date: Thu, 23 Apr 2015 12:05:00 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
MIME-Version: 1.0
References: <1428055280-12015-1-git-send-email-wency@cn.fujitsu.com>
	<1428055280-12015-2-git-send-email-wency@cn.fujitsu.com>
	<20150420153047.GB32653@stefanha-thinkpad.redhat.com>
	<5535A727.5080402@cn.fujitsu.com> <55366C81.1080202@redhat.com>
	<20150422093101.GD3882@noname.redhat.com>
	<5537742A.90504@redhat.com>
	<20150423090031.GB5289@noname.redhat.com>
	<5538B813.5090506@cn.fujitsu.com>
In-Reply-To: <5538B813.5090506@cn.fujitsu.com>
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Subject: Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's
	description
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Wen Congyang <wency@cn.fujitsu.com>, Kevin Wolf <kwolf@redhat.com>
Cc: Fam Zheng <famz@redhat.com>, Lai Jiangshan <laijs@cn.fujitsu.com>, qemu block <qemu-block@nongnu.org>, Jiang Yunhong <yunhong.jiang@intel.com>, Dong Eddie <eddie.dong@intel.com>, qemu devel <qemu-devel@nongnu.org>, Max Reitz <mreitz@redhat.com>, Gonglei <arei.gonglei@huawei.com>, Stefan Hajnoczi <stefanha@redhat.com>, Yang Hongyang <yanghy@cn.fujitsu.com>, "Dr. David Alan Gilbert" <dgilbert@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>



On 23/04/2015 11:14, Wen Congyang wrote:
> The bs->file->driver should support backing file, and use backing reference
> already.
> 
> What about the primary side? We should control when to connect to NBD server,
> not in nbd_open().

My naive suggestion could be to add a BDRV_O_NO_CONNECT option to
bdrv_open and a separate bdrv_connect callback.  Open would fail if
BDRV_O_NO_CONNECT is specified and drv->bdrv_connect is NULL.

You would then need a way to have quorum pass BDRV_O_NO_CONNECT.

Perhaps quorum is not a great match after all, and it's better to add a
new "colo" driver similar to quorum but simpler and only using the read
policy that you need for colo.  The new driver would also know how to
use BDRV_O_NO_CONNECT.  In any case the amount of work needed would not
be too big.

Paolo