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

[Jason Wang <jasowang@redhat.com>]

On 07/30/2015 04:03 PM, Dr. David Alan Gilbert wrote:
> * Dong, Eddie (eddie.dong@intel.com) 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.