Re: [Qemu-block] [PATCH v9 2/3] quorum: implement bdrv_add_child() and bdrv_del_child()

[Eric Blake]

On 01/21/2016 06:05 AM, Alberto Garcia wrote:
> On Thu 21 Jan 2016 02:54:10 AM CET, Wen Congyang <address@hidden> wrote:
> 
>>>> @@ -875,9 +878,9 @@ static int quorum_open(BlockDriverState *bs, QDict 
>>>> *options, int flags,
>>>>          ret = -EINVAL;
>>>>          goto exit;
>>>>      }
>>>> -    if (s->num_children < 2) {
>>>> +    if (s->num_children < 1) {
>>>>          error_setg(&local_err,
>>>> -                   "Number of provided children must be greater than 1");
>>>> +                   "Number of provided children must be 1 or more");
>>>>          ret = -EINVAL;
>>>>          goto exit;
>>>>      }
>>>
>>> I have a question: if you have a Quorum with just one member and you
>>> add a new one, how do you know if it has the same data as the
>>> existing one?
>>>
>>> In general, what do you do to make sure that the data in a new Quorum
>>> child is consistent with that of the rest of the array?
>>
>> Quorum can have more than one child when it starts. But we don't do
>> the similar check. So I don't think we should do such check here.
> 
> Yes, but when you start a VM you can verify in advance that all members
> of the Quorum have the same data. If you do that on a running VM how can
> you know if the new disk is consistent with the others?

User error if it is not.  Just the same as it is user error if you
request a shallow drive-mirror but the destination is not the same
contents as the backing file.  I don't think qemu has to protect us from
user error in this case.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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