Re: [Qemu-devel] Re: Strategic decision: COW format

[Avi Kivity]

On 02/23/2011 05:50 PM, Anthony Liguori wrote:
> > I still don't see.  What would you do with thousands of checkpoints?
>
>
> For reverse debugging, if you store checkpoints at a rate of save, 
> every 10ms, and then degrade to storing every 100ms after 1 second, 
> etc. you'll have quite a large number of snapshots pretty quickly.  
> The idea of snapshotting with reverse debugging is that instead of 
> undoing every instruction, you can revert to the snapshot before, and 
> then replay the instruction stream until you get to the desired point 
> in time.

You cannot replay the instruction stream since inputs (interrupts, rdtsc 
or other timers, I/O) will be different.  You need Kemari for this.

> For disaster recovery, there are some workloads that you can 
> meaningful revert to a snapshot provided that the snapshot is stored 
> at a rate of something frequency (like once a second).  Think of 
> something like a webserver where the only accumulated data is logs.  
> Losing some of the logs is better than losing all of the logs.

Are static webservers that interesting?  For disaster recovery? Anything 
else will need Kemari.

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