Re: [Qemu-block] [PATCH v12 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

[Max Reitz]

On 29.03.2016 17:50, Dr. David Alan Gilbert wrote:
> * Eric Blake (address@hidden) wrote:
>> On 03/29/2016 09:38 AM, Max Reitz wrote:
>>> On 17.03.2016 10:56, Wen Congyang wrote:
>>>> On 03/17/2016 05:48 PM, Dr. David Alan Gilbert wrote:
>>>
>>> [...]
>>>
>>>>> The children.0 notation is really confusing in the way that Berto
>>>>> describes; I hit this a couple of months ago and it really doesn't
>>>>> make sense.
>>>>
>>>> Do you mean: read from children.1 first, and then read from children.0 in
>>>> fifo mode? Yes, the behavior is very strange.
>>>
>>> So is this intended or is it not? In
>>> http://lists.nongnu.org/archive/html/qemu-block/2016-03/msg00526.html
>>> you said that it is.
>>>
>>> I myself would indeed say it is very strange. If I were a user, I would
>>> not expect this behavior. And as I developer, I think that how a BDS's
>>> child is used by its parent should solely depend on its role (e.g.
>>> whether it is "children.0" or "children.1").
>>
>> It sounds like the argument here, and in Max's thread on
>> query-block-node-tree, is that we DO have cases where order matters, and
>> so we need a way for the hot-add operation to explicitly specify where
>> in the list a child is inserted (whether it is being inserted as the new
>> primary image, or explicitly as the last resort, or somewhere in the
>> middle).  An optional parameter, that defaults to appending, may be ok,
>> but we definitely need to consider how the order of children is affected
>> by hot-add.
> 
> Certainly in the COLO case the two children are not identical; and IMHO we 
> need
> to get away from thinking about ordering and start thinking about functional
> namingd - children.0/children.1 doesn't suggest the fact they behave
> differently.

To me it does. If quorum is operating in a mode call "FIFO" I would
expect some order on the child nodes, and if the child nodes are
actually numbered in an ascending order, that is an obvious order.

Max

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