Re: [Qemu-devel] [PATCH RFC 0/4] Curling: KVM Fault Tolerance

[Orit Wasserman]

On 09/11/2013 04:54 AM, address@hidden wrote:
> Hi,
> 
>>The first is that if the VM failure happen in the middle on the live 
>>migration  >the backup VM state will be inconsistent which means you can't 
>>failover to it.
> 
> Yes, I have concerned about this problem. That is why we need a prefetch 
> buffer.
> 

You are right I missed that.

>>Solving it is not simple as you need some transaction mechanism that will 
>>>change the backup VM state only when the transaction completes (the live 
>>migration completes). >Kemari has something like that. >
> 
> The backup VM state will be loaded only when the one whole migration data is 
> prefetched. Otherwise, VM state will not be loaded. So the backup VM is 
> ensured to have a consistent state like a checkpoint.
> However, how close this checkpoint to the point of the VM failure depends on 
> the workload and bandwidth.
> 

At the moment in your implementation the prefetch buffer can be very large 
(several copies of guest memory size) 
are you planning to address this issue?

>>The second is that sadly live migration doesn't always converge this means  
>>>that the backup VM won't have a consist state to failover to. >You need to 
>>detect such a case and throttle down the guest to force convergence.
> 
> Yes, that's a problem. AFAK, qemu already have an auto convergence feature.

How about activating it when you do fault tolerance automatically?

> From another perspective,  if many migrations could not converge, maybe the 
> workload is high and the bandwidth is low,  and it is not recommended to use 
> FT in general.
> 

I agree but we need some way to notify the user of such problem.

Regards,
Orit
> 
>