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

[Jason Wang]

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?

> 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