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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: Mon, 27 Jul 2015 13:51:37 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 07/27/2015 12:49 PM, Jason Wang wrote:

On 07/27/2015 11:54 AM, Yang Hongyang wrote:

On 07/27/2015 11:24 AM, Jason Wang wrote:

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

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
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

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.

I don't get it, colo-proxy kernel module using a kthread do the
comparing and
trigger the state sync. We implemented it as a nf conntrack extension
so we need to extend the extension struct in-kernel, although it just
few lines changes to kernel, but a recompile of kernel is needed. Are you
talking about not implement it as a nf conntrack extension?

Yes, I mean implement the comparing in userspace but not in qemu.

Yes, it is an alternative, that requires other components such as
netfilter userspace tools, it will add the complexity I think, we
wanted to implement a simple solution in QEMU. Another reason is
that using other userspace tools will affect the performance, the
context switch between kernel and userspace may be an overhead.

2. We need to recompile iptables/nftables to use together with the
     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

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.

We are using tc to mirror/forward packets now. Implement in QEMU do
have some
limits, but there're also limits in kernel, if the packet do not pass
the host kernel TCP/IP stack, such as vhost-user.

But the limits are much less than userspace, no? For vhost-user, maybe
we could extend the backed to mirror the traffic also.

IMO the limits are more or less. Besides, for mirror/forward packets,
using tc requires a separate physical nic or a vlan, the nic should not
be used for other purpose. if we implement it in QEMU, using an socket
connection to forward packets, we no longer need an separate nic, it will
reduce the network topology complexity.

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.

proxy kernel module also works for secondary node.

Setup a test env is too complex. The usability is so important to a
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

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
     replication in QEMU, so with the network replication in QEMU, all
     components we needed are within QEMU, this is very important, it
     improves the usability of COLO feature! We hope it will gain more
     users and developers.

Is your block solution works for vhost?

No, it can't works for vhost and dataplane, migration also won't work
for dataplane IIRC.

5. QEMU will gain a complete VM FT solution and the most advantage FT
     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.






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