Re: [libvirt] [PATCH 1/2] snapshots: allow create --redefine to force a new configuration

On Mon, Mar 18, 2019 at 4:58 PM Peter Krempa <pkrempa(a)redhat.com&gt; wrote: Hi Peter, yeah reasonable question - let me elaborate on the case that I know. First of all this is -not- meant for --live snapshots, instead it is meant for disk snapshots - we might try to detect/limit that if you want. The other things one has to know is that we have realized that people stick way too long to their old machine types (qemu -M ...). There might be issues only solved in the newer features and in general it is recommended to use the latest type if possible. And to be fair, on the other hand it makes the maintenance burden more manageable if your delta ends at least after 10 years or so. So far we only removed some less used types and might reconsider removing the types altogether, but that would not mean it isn't recommended to update the type which still triggers the issue I try to improve here. The TL;DR of the above without stalling with even more details is, that we currently have the case where newer versions of qemu (intentionally) no more have the machine type that you saved your disk snapshot with. On a non-snapshot cases you can easily do your "virtual HW upgrade" through virsh edit and similar before you restart your guest. But for disk-snapshots there is no way anyone can modify the meta data. It is correct that the ABI checker complains (the types do differ), but then people just want to start the older disk snapshots content on a new machine type, see [1] for an example. I hope that helped to show that there are valid cases where you'd want to allow forcing changes, without any self-stabbing involved. [1]: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1802944 -- Christian Ehrhardt Software Engineer, Ubuntu Server Canonical Ltd