Re: [Qemu-devel] Some question about savem/qcow2 incremental snapshot

[Stefan Hajnoczi]

On Mon, Dec 25, 2017 at 07:54:00AM +0000, He, Junyan wrote:
> I am now focusing on snapshot optimization for Intel NVDimm kind memory. 
> Different from the normal memory, the NVDimm may be 128G, 256G or even more 
> for just one guest, and its speed is slower than the normal memory. So 
> sometimes it may take several minutes to complete just one snapshot saving. 
> Even with compression enabled, the snapshot point may consume more than 30G 
> disk space. We decide to add incremental kind snapshot saving to resolve 
> this. Just store difference between snapshot points to save time and disk 
> space.
> But the current snapshot/save_vm framework seems not to support this.
> We need to add snapshot dependency and extra operations when we LOAD and 
> DELETE the snapshot point.
> Is that possible to modify the savevm framework and add some incremental 
> snapshot support to QCOW2 format?

It sounds like you'd like to save incremental guest RAM snapshots based
on the contents of earlier snapshots.  QEMU has no way of doing this at
the moment.

In order to do this QEMU needs to keep dirty memory logging enabled at
all times so it knows which pages have been written since the last
snapshot.  This will affect guest performance.

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.

What you're proposing isn't specific to NVDIMM.  Regular RAM use cases
also benefit from incremental snapshots because less disk space is
required for the savevm data.

Some questions about your use case:

1. Does the guest have -device nvdimm?

2. Is the qcow2 file on NVDIMM?

Stefan

signature.asc
Description: PGP signature