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

[Eric Blake <eblake@redhat.com>]

On 12/25/2017 01:33 AM, He Junyan wrote:
> hi all:
> 
> 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?

In general, the list has tended to focus on external snapshots rather 
than internal; where persistent bitmaps have been the proposed mechanism 
for tracking incremental differences between snapshots.  But yes, it is 
certainly feasible that patches to improve internal snapshots to take 
advantage of incremental relationships may prove useful.  You will need 
to document all enhancements to the qcow2 file format and get that 
approved first, as interoperability demands that others reading the same 
spec would be able to interpret the image you create that is utilizing 
an internal snapshot with an incremental diff.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org