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

["Zhang, Chen" <chen.zhang@intel.com>]

> -----Original Message-----
> From: Lukas Straub [mailto:lukasstraub2@web.de]
> Sent: Saturday, August 17, 2019 2:20 AM
> To: Zhang, Chen <chen.zhang@intel.com>
> Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>; qemu-devel <qemu-
> devel@nongnu.org>; Jason Wang <jasowang@redhat.com>; Xie Changlong
> <xiechanglong.d@gmail.com>; Wen Congyang <wencongyang2@huawei.com>
> Subject: Re: [Qemu-devel] [PATCH v2 0/3] colo: Add support for continious
> replication
> 
> On Fri, 16 Aug 2019 01:51:20 +0000
> "Zhang, Chen" <chen.zhang@intel.com> wrote:
> 
> > > -----Original Message-----
> > > From: Lukas Straub [mailto:lukasstraub2@web.de]
> > > Sent: Friday, August 16, 2019 3:48 AM
> > > To: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > Cc: qemu-devel <qemu-devel@nongnu.org>; Zhang, Chen
> > > <chen.zhang@intel.com>; Jason Wang <jasowang@redhat.com>; Xie
> > > Changlong <xiechanglong.d@gmail.com>; Wen Congyang
> > > <wencongyang2@huawei.com>
> > > 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" <dgilbert@redhat.com> wrote:
> > >
> > > > * Lukas Straub (lukasstraub2@web.de) 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.
> >
> > 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?

Yes, currently we need input some net filter delete command after primary-failover.
We need make a way to remove related net-filter and chardev automatically.
But for Pacemaker it isn't a problem, you can send related qmp command after the "x-lost-heart-beat". 

Thanks
Zhang Chen

> 
> > Thanks
> > Zhang Chen
> > >
> > > Regards,
> > > Lukas Straub
> > >
> > > [1]
> > > https://github.com/ClusterLabs/resource-agents/blob/master/doc/dev-g
> > > uides/ra-dev-guide.asc#what-is-a-resource-agent