Re: Is continuous replication of state possible?

[Jacob Gorm Hansen <jacobg@diku.dk>]

Ian Pratt wrote:
>>I've been playing around with Xen a couple of months now and I am very 
>>much impressed. I am particularly found of the "live migration" feature. 
>>I was wondering if it is possible to make a instance continuously 
>>replicate the state of another instance and then make the other instance 
>>run if the original instance fails.
> 
> 
> Software-implemented hardware fault-tolerance is on the Xen
> research roadmap.
> 
> It basically just requires deterministic execution and event
> injection. Doing this for uniprocessor guests is fairly straight
> forward. Doing it for SMP guests (with decent performance) is
> going to be a huge challenge, as determinism is hard to achieve. We're
> looking in to it...

I did a little reading on this subject a couple of years back, and it 
seems that on Pentiums getting deterministic execution is impossible 
even for UPs, as long as you allow preemptive multitasking. Because 
(according to the Intel manuals) the precision of the Pentium 
performance counters cannot be relied on, the timer and other interrupts 
will essentially act as a random generator. Naturally you can do peridic 
checkpointing, but there will be no way correctness can be guaranteed, 
unless you coordinate all outgoing traffic between replicas before 
making it visible to the outside world.

There is a paper by Bressoud and Schneider about hypervisor-based fault 
tolerance on the PA-RISC (which had precise performance counters) which 
is worth reading, I found a copy online at 
http://roc.cs.berkeley.edu/294fall01/readings/bressoud.pdf .

I think is more likely to work at a higher level, when you know the 
semantics of your application, Dmitrii Zagorodnov did some work on that 
and reported good results, see for instance 
http://ieeexplore.ieee.org/iel5/8589/27228/01209950.pdf .

Best regards,
Jacob


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