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

From: Dr. David Alan Gilbert
Subject: Re: [Qemu-block] COLO: how to flip a secondary to a primary?
Date: Mon, 25 Jan 2016 20:20:02 +0000
User-agent: Mutt/1.5.24 (2015-08-30)

* Li Zhijian (address@hidden) 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

> >Hailiang or Zhijian can answer this question.
> Thanks
> Li Zhijian
Dr. David Alan Gilbert / address@hidden / Manchester, UK

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