Re: [Qemu-devel] [POC] colo-proxy in qemu

[Jason Wang <jasowang@redhat.com>]

On 07/24/2015 04:04 PM, Yang Hongyang wrote:
> Hi Jason,
>
> On 07/24/2015 10:12 AM, Jason Wang wrote:
>>
>>
>> On 07/24/2015 10:04 AM, Dong, Eddie wrote:
>>> Hi Stefan:
>>>     Thanks for your comments!
>>>
>>>> On Mon, Jul 20, 2015 at 02:42:33PM +0800, Li Zhijian wrote:
>>>>> We are planning to implement colo-proxy in qemu to cache and compare
>>>> packets.
>>>>
>>>> I thought there is a kernel module to do that?
>>>     Yes, that is the previous solution the COLO sub-community choose
>>> to go, but we realized it might be not the best choices, and thus we
>>> want to bring discussion back here :)  More comments are welcome.
>>>
>>
>> Hi:
>>
>> Could you pls describe more details on this decision? What's the reason
>> that you realize it was not the best choice?
>
> Below is my opinion:
>
> We realized that there're disadvantages do it in kernel spaces:
> 1. We need to recompile kernel: the colo-proxy kernel module is
>    implemented as a nf conntrack extension. Adding a extension need to
>    modify the extension struct in-kernel, so recompile kernel is needed.

There's no need to do all in kernel, you can use a separate process to
do the comparing and trigger the state sync through monitor.

> 2. We need to recompile iptables/nftables to use together with the
> colo-proxy
>    kernel module.
> 3. Need to configure primary host to forward input packets to
> secondary as
>    well as configure secondary to forward output packets to primary
> host, the
>    network topology and configuration is too complex for a regular user.
>

You can use current kernel primitives to mirror the traffic of both PVM
and SVM to another process without any modification of kernel. And qemu
can offload all network configuration to management in this case.  And
what's more import, this works for vhost. Filtering in qemu won't work
for vhost.


> You can refer to http://wiki.qemu.org/Features/COLO
> to see the network topology and the steps to setup an env.

The figure "COLO Framework" shows there's a proxy kernel module in
primary node but in secondary node this is done through a process? This
will complicate the environment a bit more.

>
> Setup a test env is too complex. The usability is so important to a
> feature
> like COLO which provide VM FT solution, if fewer people can/willing to
> setup the env, the feature is useless. So we decide to develop user space
> colo-proxy.

If the setup is too complex, need to consider to simplify or reuse codes
and designs. Otherwise you probably introduce something new that needs
fault tolerance.

>
> The advantage is obvious,
> 1. we do not need to recompile kernel.
> 2. No need to recompile iptables/nftables.

As I descried above, looks like there's no need to modify kernel.

> 3. we do not need to deal with the network configuration, we just using a
>    socket connection between 2 QEMUs to forward packets.

All network configurations should be offloaded to management. And you
still need a dedicated topology according to the wiki.

> 4. A complete VM FT solution in one go, we have already developed the
> block
>    replication in QEMU, so with the network replication in QEMU, all
>    components we needed are within QEMU, this is very important, it
> greatly
>    improves the usability of COLO feature! We hope it will gain more
> testers,
>    users and developers.

Is your block solution works for vhost?

> 5. QEMU will gain a complete VM FT solution and the most advantage FT
> solution
>    so far!
>
> Overall, usability is the most important factor that impact our choice.
>
>

Usability will be improved if you can use exist primitives and decouple
unnecessary codes from qemu.

Thanks

>>
>> Thanks
>> .
>>
>