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Date: Thu, 23 Apr 2015 19:35:49 +0800
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Subject: Re: [Qemu-devel] [PATCH COLO v3 01/14] docs: block replication's
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To: Paolo Bonzini <pbonzini@redhat.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 04/23/2015 06:44 PM, Paolo Bonzini wrote:
> 
> 
> On 23/04/2015 12:40, Kevin Wolf wrote:
>> The question that is still open for me is whether it would be a colo.c
>> or an active-mirror.c, i.e. if this would be tied specifically to COLO
>> or if it could be kept generic enough that it could be used for other
>> use cases as well.
> 
> Understood (now).
> 
>>>> What I think is really needed here is essentially an active mirror
>>>> filter.
>>>
>>> Yes, an active synchronous mirror.  It can be either a filter or a
>>> device.  Has anyone ever come up with a design for filters?  Colo
>>> doesn't need much more complexity than a "toy" blkverify filter.
>>
>> I think what we're doing now for quorum/blkverify/blkdebug is okay.
>>
>> The tricky and yet unsolved part is how to add/remove filter BDSes at
>> runtime (dynamic reconfiguration), but IIUC that isn't needed here.
> 
> Yes, it is.  The "defer connection to NBD when replication is started"
> is effectively "add the COLO filter" (with the NBD connection as a
> children) when replication is started.
> 
> Similarly "close the NBD device when replication is stopped" is
> effectively "remove the COLO filter" (which brings the NBD connection
> down with it).

Hmm, I don't understand it clearly. Do you mean:
1. COLO filter is quorum's child
2. We can add/remove quorum's child at run-time.

If I misunderstand something, please correct me.

Thanks
Wen Congyang

> 
> Paolo
> .
>