Re: [Qemu-devel] COLO: how to flip a secondary to a primary?

[Li Zhijian <lizhijian@cn.fujitsu.com>]

On 01/26/2016 04:20 AM, Dr. David Alan Gilbert wrote:
> * Li Zhijian (lizhijian@cn.fujitsu.com) wrote:
>>
>>
>> On 01/25/2016 09:32 AM, Wen Congyang wrote:
>>>>>     f) I've not thought about the colo-proxy that much yet - I guess that
>>>>>        existing connections need to keep their sequence number offset but
>>
>> Strictly speaking， after failover, we only need to keep servicing for the tcp connections which are
>> established after the last checkpoint but not all existing connections. Because after a checkpoint
>> （primary and secondary node works well）, primary vm and secondary vm is same， that means the existing
>> tcp connection has the same sequence。
>>
>>>>>        new connections made by what is now the primary dont need to do anything
>>>>>        special.
>> Yes, you are right.
>
> I wonder whether we need to do something special to the new-secondary;
> consider this:
>
>     1 primary (P1) & secondary (S1) run together
>     2 New connection opened
>     3    secondary records an offset
>     4 <running OK for a while - no checkpoint>
>     5 primary (P1) fails; do failover to secondary
>     6 secondary (S1) still rewrites sequence for connection opened at (2)
>     7 Start new-secondary (S2), send checkpoint from S1->S2
>     8 S2 has same guest contents as S1; so the
>       sequence numbers are still offset compared to the outside world.
>
> So S2 needs to be sent the offsets for existing connections, otherwise
> is S1 was then to fail, S2 would send the wrong output on the existing
> connection?

Thanks for the example.
Sure, if we support continuous FT, colo proxy need to implement migration_save and migration_load.
At the beginning of (7), we need to save colo_proxy info(including connection info and sequence offset) at S1
and load colo_proxy at S2. S1/S2 need to keep doing tcp re-writer for the connections opened at (2)
until they are closed.

Thanks
Li Zhijian

>
> Dave
>
>>
>>
>>> Hailiang or Zhijian can answer this question.
>>
>>
>> Thanks
>> Li Zhijian
>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>
>
> .
>

-- 
Best regards.
Li Zhijian (8555)