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From: Wen Congyang <wency@cn.fujitsu.com>
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Date: Mon, 7 Sep 2015 13:53:25 +0800
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Subject: Re: [Qemu-devel] [PATCH v4 00/38] blockdev: BlockBackend and media
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To: Eric Blake <eblake@redhat.com>, Max Reitz <mreitz@redhat.com>, qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Alberto Garcia <berto@igalia.com>, qemu-devel@nongnu.org, Markus Armbruster <armbru@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, John Snow <jsnow@redhat.com>

On 09/02/2015 11:02 PM, Eric Blake wrote:
> On 07/20/2015 11:45 AM, Max Reitz wrote:
>> First of all: Thank you, Eric and Berto, for reviewing v3! And thank
>> you, Fam, for at least having a peek at it and being confident enough to
>> base a series of your own on it. :-)
>>
>> This series reworks a lot regarding BlockBackend and media. Basically,
>> it allows empty BlockBackends, that is BBs without a BDS tree.
>>
>> Before this series, empty drives are represented by a BlockBackend with
>> an empty BDS attached to it (a BDS with a NULL driver). However, now we
>> have BlockBackends, thus an empty drive should be represented by a
>> BlockBackend without any BDS tree attached to it. This is what this
>> series does.
> 
> We've obviously missed 2.4; but now that 2.5 is open, are there plans to
> get this series in sooner rather than later?  Wen's work on dynamically
> adding/deleting children from a quorum would benefit from the ability to
> create a BDS without an associated BB, which in turn would help the COLO
> work.
> 

This feature is very useful for COLO. COLO needs the following:
1. adding/deleting children from a quorum dynamically
2. allow references for backing files

The BDS without BB only can be created by the monitor command blockdev-add.
How to create it in the command line?

Thanks
Wen Congyang