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

[Alberto Garcia]

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?

Berto