Re: [Qemu-devel] [RFC PATCH v2 00/10] Add colo-proxy based on netfilter

[Dr. David Alan Gilbert]

* Jason Wang (address@hidden) wrote:
> 
> 
> On 01/04/2016 04:16 PM, Zhang Chen wrote:
> >
> >
> > On 01/04/2016 01:37 PM, Jason Wang wrote:
> >>
> >> On 12/31/2015 04:40 PM, Zhang Chen wrote:
> >>>
> >>> On 12/31/2015 10:36 AM, Jason Wang wrote:
> >>>> On 12/22/2015 06:42 PM, Zhang Chen wrote:
> >>>>> From: zhangchen <address@hidden>
> >>>>>
> >>>>> Hi,all
> >>>>>
> >>>>> This patch add an colo-proxy object, COLO-Proxy is a part of COLO,
> >>>>> based on qemu netfilter and it's a plugin for qemu netfilter. the
> >>>>> function
> >>>>> keep Secondary VM connect normal to Primary VM and compare packets
> >>>>> sent by PVM to sent by SVM.if the packet difference,notify COLO do
> >>>>> checkpoint and send all primary packet has queued.
> >>>> Thanks for the work. I don't object this method but still not
> >>>> convinced
> >>>> that qemu is the best place for this.
> >>>>
> >>>> As been raised in the past discussion, it's almost impossible to
> >>>> cooperate with vhost backends. If we want this to be used in
> >>>> production
> >>>> environment, need to think of a solution for vhost. There's no such
> >>>> worry if we decouple this from qemu.
> >>>>
> >>>>> You can also get the series from:
> >>>>>
> >>>>> https://github.com/zhangckid/qemu/tree/colo-v2.2-periodic-mode-with-colo-proxyV2
> >>>>>
> >>>>>
> >>>>>
> >>>>> Usage:
> >>>>>
> >>>>> primary:
> >>>>> -netdev tap,id=bn0 -device e1000,netdev=bn0
> >>>>> -object
> >>>>> colo-proxy,id=f0,netdev=bn0,queue=all,mode=primary,addr=host:port
> >>>>>
> >>>>> secondary:
> >>>>> -netdev tap,id=bn0 -device e1000,netdev=bn0
> >>>>> -object
> >>>>> colo-proxy,id=f0,netdev=bn0,queue=all,mode=secondary,addr=host:port
> >>>> Have a quick glance at how secondary mode work. What it does is just
> >>>> forwarding packets between a nic and a socket, qemu socket backend did
> >>>> exact the same job. You could even use socket in primary node and let
> >>>> packet compare module talk to both primary and secondary node.
> >>> If we use qemu socket backend , the same netdev will used by qemu
> >>> socket and
> >>> qemu netfilter. this will against qemu net design. and then, when colo
> >>> do failover,
> >>> secondary do not have backend to use. that's the real problem.
> >> Then, maybe it's time to implement changing the netdev of a nic. The
> >> point here is that what secondary mode did is in fact a netdev backend
> >> instead of a filter ...
> >
> > Currently, you are right. in colo-proxy V2 code, I just compare IP
> > packet to
> > decide whether to do checkpoint.
> > But, in colo-proxy V3 I will compare tcp,icmp,udp packet to decide it.
> > because that can reduce frequency of checkpoint and improve
> > performance. To keep tcp connection well, colo secondary need to record
> > primary guest's init seq and adjust secondary guest's ack. if colo do
> > failover,
> > secondary also need do this to old tcp connection. qemu socket
> > can't do this job.
> 
> So a question here: is it a must to do things (e.g TCP analysis stuffs)
> at secondary? Looks like we could do this at primary node. And I saw
> you're doing packet comparing in primary node, any advantages of doing
> this in primary instead of secondary?

It needs to do this on the secondary; the trick is that things like TCP sequence
numbers are likely to be different on the primary and secondary; the kernel 
colo-proxy
implementation solved this problem by rewriting the sequence numbers on
the secondary to match the primary, after a failover, the secondary has
to keep doing that rewrite to ensure existing connections are OK.
Thus it's holding some state about the current connections.
I think also, to be able to do a 2nd failover (i.e. recover from the 1st failure
and then sometime later have another) you'd have to sync this
state over to a new host, so again that says the state needs to be part of
qemu or at least easily available to it.

Dave

> > and another problem is do failover, if we use qemu socket
> > to be backend in secondary, when colo do failover, I don't know how to
> > change
> > secondary be a normal qemu, if you know, please tell me.
> 
> Current qemu couldn't do this, but I mean we implement something like
> nic_change_backend which can change nic's peer(s). With this, in
> secondary, we can replace the socket backend with whatever you want (e.g
> tap or other).
> 
> Thanks
> 
> >
> >
> > Thanks for your revew
> > zhangchen 
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK