Re: [Qemu-devel] [PATCH v2 0/3] colo: Add support for continious replication

[Dr. David Alan Gilbert]

* Lukas Straub (address@hidden) wrote:
> On Fri, 16 Aug 2019 01:51:20 +0000
> "Zhang, Chen" <address@hidden> wrote:
> 
> > > -----Original Message-----
> > > From: Lukas Straub [mailto:address@hidden]
> > > Sent: Friday, August 16, 2019 3:48 AM
> > > To: Dr. David Alan Gilbert <address@hidden>
> > > Cc: qemu-devel <address@hidden>; Zhang, Chen
> > > <address@hidden>; Jason Wang <address@hidden>; Xie
> > > Changlong <address@hidden>; Wen Congyang
> > > <address@hidden>
> > > Subject: Re: [Qemu-devel] [PATCH v2 0/3] colo: Add support for continious
> > > replication
> > >
> > > On Thu, 15 Aug 2019 19:57:37 +0100
> > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> > >
> > > > * Lukas Straub (address@hidden) wrote:
> > > > > Hello Everyone,
> > > > > These Patches add support for continious replication to colo.
> > > > > Please review.
> > > >
> > > >
> > > > OK, for those who haven't followed COLO for so long; 'continuous
> > > > replication' is when after the first primary fails, you can promote
> > > > the original secondary to a new primary and start replicating again;
> > > >
> > > > i.e. current COLO gives you
> > > >
> > > > p<->s
> > > >     <primary fails>
> > > >     s
> > > >
> > > > with your patches you can do
> > > >
> > > >     s becomes p2
> > > >     p2<->s2
> > > >
> > > > and you're back to being resilient again.
> > > >
> > > > Which is great; because that was always an important missing piece.
> > > >
> > > > Do you have some test scripts/setup for this - it would be great to
> > > > automate some testing.
> > >
> > > My Plan is to write a Pacemaker Resource Agent[1] for qemu-colo and then 
> > > do
> > > some long-term testing in my small cluster here. Writing standalone tests 
> > > using
> > > that Resource Agent should be easy, it just needs to be provided with the 
> > > right
> > > arguments and environment Variables.

Could you update tests/test-replication.c to test the extra steps?

Dave

> > Thanks Dave's explanation.
> > It looks good for me and I will test this series in my side.
> >
> > Another question: Is "Pacemaker Resource Agent[1] "  like a heartbeat 
> > module?
> 
> It's a bit more than that. Pacemaker itself is an Cluster Resource Manager, 
> you can think of it like sysvinit but for clusters. It controls where in the 
> cluster Resources run, what state (master/slave) and what to do in case of a 
> Node or Resource failure. Now Resources can be anything like SQL-Server, 
> Webserver, VM, etc. and Pacemaker itself doesn't directly control them, 
> that's the Job of the Resource Agents. So a Resource Agent is like an 
> init-script, but cluster-aware with more actions like start, stop, monitor, 
> promote (to master) or migrate-to.
> 
> > I have wrote an internal heartbeat module running on Qemu, it make COLO can 
> > detect fail and trigger failover automatically, no need external APP to 
> > call the QMP command "x-colo-lost-heartbeat". If you need it, I can send a 
> > RFC version recently.
> 
> Cool, this should be faster to failover than with Pacemaker.
> What is the plan with cases like Primary-failover, which need to issue 
> multiple commands?
> 
> > Thanks
> > Zhang Chen
> > >
> > > Regards,
> > > Lukas Straub
> > >
> > > [1] 
> > > https://github.com/ClusterLabs/resource-agents/blob/master/doc/dev-guides/ra-dev-guide.asc#what-is-a-resource-agent
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK