Re: [Qemu-devel] [RFC] Intermediate block mirroring

[Max Reitz <mreitz@redhat.com>]

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On 2018-04-25 15:42, Alberto Garcia wrote:
> On Wed 25 Apr 2018 03:06:34 PM CEST, Max Reitz wrote:
>>>>>>>>> One other way is to have a more generic replace-node command
>>>>>>>>> which would call bdrv_replace_node(), but I don't know if we
>>>>>>>>> want to expose that and I don't have any other use case for it
>>>>>>>>> at the moment.
>>>>>>>>
>>>>>>>> I think we do want to expose that.  As far as I'm informed, for
>>>>>>>> graph manipulation we want a command that can replace nodes
>>>>>>>> (including replacing nothing by a new node or replacing an
>>>>>>>> existing node by nothing, if the parent supports that).
>>>>>>>
>>>>>>> Was there any discussion about this in the mailing list?
>>>>>>> (proposed name for this function, features, etc.)
>>>>>>
>>>>>> Well, there is x-blockdev-change.  But we probably want to expose
>>>>>> bdrv_reopen() in the long run.
>>>>>
>>>>> Exposing bdrv_reopen() itself shouldn't be too much work (it takes
>>>>> just two node names, or am I missing something?).
>>>
>>> I just realized that I meant "Exposing bdrv_replace_node()" here.
>>>
>>>> So from my perspective...  If you think it's easy, why don't you try
>>>> it and then we'll see? *cough*
>>>
>>> I'm doing it :-)
>>>
>>>>> There's still the question of how to update the backing file string
>>>>> (on the overlay). stream and commit do it automatically, but if we do
>>>>> bdrv_reopen() or the aforementioned modification to blockdev-mirror
>>>
>>> bdrv_replace_node() again
>>
>> But the question stands whether we need simple node replacement when
>> we want bdrv_reopen() anyway.  In addition, we don't need just
>> replacement, we also need addition and removal (e.g. for backing files
>> or quorum children) -- and especially in the case of quorum, that is
>> going to be a pain (mostly naming the children).
>>
>> With bdrv_reopen(), we can just require the user to respecify all
>> children, so we don't run into the issue of how to name things at
>> least.
> 
> So in this example, if you want to replace [B] with [E] you would reopen
> [C] specifying the new backing file?
> 
>    [A] <- [B] <- [C] <- [D]
> 
>           [E]

You can just use child node name references like in blockdev-add.

Max


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