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From: Stefan Hajnoczi <stefanha@gmail.com>
Date: Wed, 10 Jan 2018 20:17:11 +0000
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Subject: Re: [Qemu-devel] Some question about savem/qcow2 incremental
 snapshot
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To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: "He, Junyan" <junyan.he@intel.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Juan Quintela <quintela@redhat.com>, John Snow <jsnow@redhat.com>

On Wed, Jan 10, 2018 at 8:15 PM, Dr. David Alan Gilbert
<dgilbert@redhat.com> wrote:
> * Stefan Hajnoczi (stefanha@gmail.com) wrote:
>> On Tue, Jan 9, 2018 at 7:55 PM, Dr. David Alan Gilbert
>> <dgilbert@redhat.com> wrote:
>> >> Certain guest operations like rebooting or zeroing memory will defeat
>> >> the incremental guest RAM snapshot feature.  It's worth thinking about
>> >> these cases to make sure this feature would be worth it in real use
>> >> cases.
>> >
>> > But those probably wouldn't upset an NVDimm?
>>
>> If the guest dirties all RAM then the incremental snapshot feature
>> degrades to a full snapshot.  I'm asking if there are common
>> operations where that happens.
>>
>> I seem to remember Windows guests zero all pages on cold boot.  Maybe
>> that's not the case anymore.
>>
>> Worth checking before embarking on this feature because it could be a
>> waste of effort if it turns out real-world guests dirty all memory in
>> common cases.
>
> Right, but I'm hoping that there's some magic somewhere where an NVDimm doesn't
> get zero'd because of a cold boot since that would seem to make it
> volatile.

This feature isn't specific to NVDIMM though.  It would be equally
useful for regular RAM.

Stefan