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Date: Thu, 18 Jun 2015 19:40:16 +0800
From: Wen Congyang <wency@cn.fujitsu.com>
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Subject: Re: [Qemu-devel] [PATCH COLO-Block v6 04/16] block: Parse
 "backing_reference" option to reference existing BDS
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To: Stefan Hajnoczi <stefanha@gmail.com>, Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@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>, Paolo Bonzini <pbonzini@redhat.com>, Yang Hongyang <yanghy@cn.fujitsu.com>, "Dr. David
	Alan Gilbert" <dgilbert@redhat.com>, zhanghailiang <zhang.zhanghailiang@huawei.com>

On 06/18/2015 06:50 PM, Stefan Hajnoczi wrote:
> On Thu, Jun 18, 2015 at 04:49:09PM +0800, Wen Congyang wrote:
>> Usage:
>> -drive file=xxx,id=Y, \
>> -drive file=xxxx,id=X,backing_reference.drive_id=Y,backing_reference.hidden-disk.*
>>
>> It will create such backing chain:
>>                {virtio-blk dev 'Y'}                                      {virtio-blk dev 'X'}
>>                          |                                                          |
>>                          |                                                          |
>>                          v                                                          v
>>
>>     [base] <- [mid] <- ( Y )  <----------------- (hidden target) <--------------- ( X )
>>
>>                          v                              ^
>>                          v                              ^
>>                          v                              ^
>>                          v                              ^
>>                          >>>> drive-backup sync=none >>>>
>>
>> X's backing file is hidden-disk, and hidden-disk's backing file is Y.
>> Disk Y may be opened or reopened in read-write mode, so A block backup
>> job is automatically created: source is Y and target is hidden disk.
>> Active disk X, hidden disk, and Y are all on the same AioContext.
> 
> The command-line option name confused me.  I expected "backing_reference" to result in:
> 
>   Y <- X

So if I want to create
Y <- hidden-disk <- X
The command line can be
backing.file.filename=xxx,backing.driver=xxx,backing.backing.backing_reference=Y

> 
> The backup block job and hidden target makes this option more than just
> referencing the backing device.
> 
> The name "backing.backup-reference" is clearer to me, but I think Kevin
> might have more idea for you on this patch.
> 

Any suggestion is welcome.

Thanks
Wen Congyang