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Date: Wed, 22 Apr 2015 17:28:01 +0800
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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: Stefan Hajnoczi <stefanha@gmail.com>, 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>

On 04/22/2015 05:18 PM, Stefan Hajnoczi wrote:
> On Tue, Apr 21, 2015 at 05:28:01PM +0200, Paolo Bonzini wrote:
>> 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?
> 
> Why does NBD need to know about COLO?  It should be possible to use
> iSCSI or other protocols too.

Hmm, if you want to use iSCSI or other protocols, you should update the driver
to implement block replication's control interface.

Currently, we only support nbd now.

Thanks
Wen Congyang

> 
> Stefan
>