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Date: Thu, 23 Apr 2015 12:17:16 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's
	description
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To: Paolo Bonzini <pbonzini@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>

Am 23.04.2015 um 12:05 hat Paolo Bonzini geschrieben:
> 
> 
> 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().

Why do you need to create the block device before the connection should
be made?

> 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.

Please don't add new flags. If we have to, we can introduce a new option
(in the QDict), but first let's check if it's really necessary.

> 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.

I thought the same, but haven't looked at the details yet. But if I
understand correctly, the plan is to take quorum and add options to turn
off the functionality of using a quorum - that's a bit odd.

What I think is really needed here is essentially an active mirror
filter.

Kevin