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Date: Wed, 22 Apr 2015 11:31:01 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [Qemu-block] [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>, Stefan Hajnoczi <stefanha@gmail.com>, 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 21.04.2015 um 17:28 hat Paolo Bonzini geschrieben:
> 
> 
> On 21/04/2015 03:25, Wen Congyang wrote:
> >> > Please do not introduce "<name>+colo" block drivers.  This approach is
> >> > invasive and makes block replication specific to only a few block
> >> > drivers, e.g. NBD or qcow2.
> > NBD is used to connect to secondary qemu, so it must be used. But the primary
> > qemu uses quorum, so the primary disk can be any format.
> > The secondary disk is nbd target, and it can also be any format. The cache
> > disk(active disk/hidden disk) is an empty disk, and it is created before run
> > COLO. The cache disk format is qcow2 now. In theory, it can be ant format which
> > supports backing file. But the driver should be updated to support colo mode.
> > 
> > > A cleaner approach is a QMP command or -drive options that work for any
> > > BlockDriverState.
> > 
> > OK, I will add a new drive option to avoid use "<name>+colo".
> 
> Actually I liked the "foo+colo" names.
> 
> These are just internal details of the implementations and the
> primary/secondary disks actually can be any format.
> 
> Stefan, what was your worry with the +colo block drivers?

I haven't read the patches yet, so I may be misunderstanding, but
wouldn't a separate filter driver be more appropriate than modifying
qcow2 with logic that has nothing to do with the image format?

Kevin