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

From: Yang Hongyang
Subject: Re: [Qemu-devel] [POC] colo-proxy in qemu
Date: Fri, 24 Jul 2015 16:04:38 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

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

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.


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

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

The advantage is obvious,
1. we do not need to recompile kernel.
2. No need to recompile iptables/nftables.
3. we do not need to deal with the network configuration, we just using a
   socket connection between 2 QEMUs to forward packets.
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.
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.



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