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

[Gonglei]

On 2015/7/30 12:23, Jason Wang wrote:
> 
> 
> On 07/20/2015 02:42 PM, Li Zhijian wrote:
>> Hi, all
>>
>> We are planning to implement colo-proxy in qemu to cache and compare
>> packets.
>> This module is one of the important component of COLO project and now
>> it is
>> still in early stage, so any comments and feedback are warmly welcomed,
>> thanks in advance.
>>
>> ## Background
>> COLO FT/HA (COarse-grain LOck-stepping Virtual Machines for Non-stop
>> Service)
>> project is a high availability solution. Both Primary VM (PVM) and
>> Secondary VM
>> (SVM) run in parallel. They receive the same request from client, and
>> generate
>> responses in parallel too. If the response packets from PVM and SVM are
>> identical, they are released immediately. Otherwise, a VM checkpoint
>> (on demand)
>> is conducted.
>> Paper:
>> http://www.socc2013.org/home/program/a3-dong.pdf?attredirects=0
>> COLO on Xen:
>> http://wiki.xen.org/wiki/COLO_-_Coarse_Grain_Lock_Stepping
>> COLO on Qemu/KVM:
>> http://wiki.qemu.org/Features/COLO
>>
>> By the needs of capturing response packets from PVM and SVM and
>> finding out
>> whether they are identical, we introduce a new module to qemu
>> networking called
>> colo-proxy.
> 
> A question here, the packet comparing may be very tricky. For example,
> some protocol use random data to generate unpredictable id or something
> else. One example is ipv6_select_ident() in Linux. So COLO needs a
> mechanism to make sure PVM and SVM can generate same random data?
> 
Good question, the random data connection is a big problem for COLO. At
present, it will trigger checkpoint processing because of the different random 
data.
I don't think any mechanisms can assure two different machines generate
the same random data. If you have any ideas, pls tell us :)

Frequent checkpoint can handle this scenario, but maybe will cause the
performance poor. :(

Regards,
-Gonglei