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

[Jason Wang]

On 07/30/2015 04:03 PM, Dr. David Alan Gilbert wrote:
> * Dong, Eddie (address@hidden) wrote:
>>>> 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. :(
>>>
>> The assumption is that, after VM checkpoint, SVM and PVM have identical 
>> internal state, so the pattern used to generate random data has high 
>> possibility to generate identical data at short time, at least...
> They do diverge pretty quickly though; I have simple examples which
> reliably cause a checkpoint because of simple randomness in applications.
>
> Dave
>

And it will become even worse if hwrng is used in guest.