Re: [Qemu-devel] [PATCH v4 00/38] blockdev: BlockBackend and media

[Max Reitz <mreitz@redhat.com>]

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On 07.09.2015 07:53, Wen Congyang wrote:
> 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?

Right now: Not at all. Of course it would be possible to add a new
command line parameter ("-blockdev"? "-bds"?) or option to drive
("backend=off" or whatever), but right now I didn't see a need for that.

This series uses this so you can add a new medium for e.g. a CD drive at
runtime. The backends remains attached to the drive, and you swap out
the BDS tree. Thus, there was no need to add a medium via the command line.

Furthermore, often runtime configuration is a subset of command line
configuration, i.e. you can always start the VM paused, invoke some QMP
commands, and then continue the VM. Therefore I don't know whether we
really need the ability to add BB-less BDS trees via the command line.

Max


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