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

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

On 03/17/2016 07:25 PM, Dr. David Alan Gilbert wrote:
> * Wen Congyang (wency@cn.fujitsu.com) wrote:
>> On 03/17/2016 06:07 PM, Alberto Garcia wrote:
>>> On Thu 17 Mar 2016 10:56:09 AM CET, Wen Congyang wrote:
>>>>> We should have the failure modes documented, and how you'll use it
>>>>> after failover etc Without that it's really difficult to tell if this
>>>>> naming is right.
>>>>
>>>> For COLO, children.0 is the real disk, children.1 is replication
>>>> driver.  After failure, children.1 will be removed by the user. If we
>>>> want to continue do COLO, we need add a new children.1 again.
>>>
>>> What if children.0 fails ?
>>
>> For COLO, reading from children.1 always fails. if children.0 fails, it
>> means that reading from the disk fails. The guest vm will see the I/O error.
> 
> How do we get that to cause a fail over before the guest detects it?
> If the primary's local disk (children.0) fails then if we can failover
> at that point then the guest carries running on the secondary without
> ever knowing about the failure.

COLO is not designed for such case. The children.0 can also be quorum, so
you can add more than one real disk, and get more reliability. Another
choice is that, the real disk is an external storage, and it has
its own replication solution.

COLO is designed for such case: the host is crashed, and the guest is still
alive after failover, the client doesn't know this event.

Thanks
Wen Congyang

> 
> Dave
> 
>>
>> Thanks
>> Wen Congyang
>>
>>>
>>> Berto
>>>
>>>
>>> .
>>>
>>
>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> 
> .
>