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Date: Wed, 23 Feb 2011 10:04:38 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: Strategic decision: COW format
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To: Avi Kivity <avi@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Chunqiang Tang <ctang@us.ibm.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Markus Armbruster <armbru@redhat.com>

On 02/23/2011 10:03 AM, Avi Kivity wrote:
> 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.

Yes, I'm well aware of this.  I don't think all the pieces where ever 
really there to do this.

Regards,

Anthony Liguori

>>
>> 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.
>