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

[Wen Congyang <wency@cn.fujitsu.com>]

On 03/16/2016 08:38 PM, Alberto Garcia wrote:
> On Mon 14 Mar 2016 07:02:08 AM CET, Changlong Xie <xiecl.fnst@cn.fujitsu.com> 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.

> 
> - How do you obtain the child name?

IIRC, the answer is no now. I think we can improve 'info block' output

> 
> - 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.

Thanks
Wen Congyang

> 
> I also think that it would be great to have tests for this
> functionality, but they can be added later.
> 
> Thanks,
> 
> Berto
> 
> 
> .
>