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

[Wen Congyang]

On 03/17/2016 05:10 PM, Alberto Garcia wrote:
> On Thu 17 Mar 2016 02:22:40 AM CET, Wen Congyang <address@hidden> wrote:
>>>>>> @@ -81,6 +82,8 @@ typedef struct BDRVQuorumState {
>>>>>>       bool rewrite_corrupted;/* true if the driver must rewrite-on-read 
>>>>>> corrupted
>>>>>>                               * block if Quorum is reached.
>>>>>>                               */
>>>>>> +    unsigned long *index_bitmap;
>>>>
>>>> Hi Berto
>>>>
>>>> *NOTE*, In the old version, we just used "bs->node_name", but in the
>>>> lastest one, as Kevin suggested we introduce
>>>> "child->child_name"(formart as "children.xxx"), this is the key cause
>>>> why we need this two functions here.
>>>
>>> I'm sorry I missed this discussion earlier. Your code seems technically
>>> correct but I have several questions:
>>>
>>> - I read that one of the reasons for this change is that "In theory, the
>>>   same node could be attached twice to the same parent in different
>>>   roles.". Is there any example of that? What's the use case?
>>
>> Kevin may know the case.
> 
> Kevin, do you have an example?
> 
>>> - How do you obtain the child name?
>>
>> IIRC, the answer is no now. I think we can improve 'info block' output
> 
> Okay, but then we should extend that first, otherwise this API cannot be
> used.
> 
>>> - I see that if you have children.0 and children.1 (let's say hd0.qcow2
>>>   and hd1.qcow2), then you remove children.0 and add it again, it will
>>>   keep the 'children.0' name (that's what the bitmap is for if I'm
>>>   understanding it correctly). However the position in the s->children
>>>   array will change because you do memmove() when you remove children.0
>>>   and then add it again to the end of the array.
>>>
>>>   Initial status:
>>>
>>>     s->children[0] <--> "children.0" (hd0.qcow2)
>>>     s->children[1] <--> "children.1" (hd1.qcow2)
>>>
>>>   children.0 (hd0.qcow2) is removed:
>>>
>>>     s->children[0] <--> "children.1" (hd1.qcow2)
>>>
>>>   children.0 (hd0.qcow2) is added again:
>>>
>>>     s->children[0] <--> "children.1" (hd1.qcow2)
>>>     s->children[1] <--> "children.0" (hd0.qcow2)
>>
>> Yes, it is correct.
>>
>>>
>>>   Is this correct? Is this the indented behavior? Since you are reading
>>>   in FIFO mode, now hd1.qcow2 will always be read first, so if
>>>   children.1 was the secondary disk, it has just become the primary.
>>
>> Yes.
> 
> And don't you need a way to control the order in which the disks must be
> read for COLO?

I think in fifo mode, we should read the disk first that is added earlier.

We don't need a way to control the order now.

Thanks
Wen Congyang

> 
> Berto
> 
> 
> .
>