[pve-devel] [PATCH qemu-server] fix #3581: pass size via argument for memory-backend-ram qmp call

[Wolfgang Bumiller]

On Tue, Aug 17, 2021 at 05:32:00PM +0200, Constantin Herold wrote:
> Sure no problem.
> 
> In that case the patch will probably break memory hotplug for any VM's
> tagged to Qemu <=4.2

I think it's okay as it's just the command interface to the running qemu
process.

* pve6 has qemu 5.2 (supports both variants)
* pve7 has qemu 6.0 (supports only the new one)

For clusters we recommend migrating running machines to different nodes
when upgrading, which means they'll be running on pve6+qemu5.2 and then get
migrated to pve7-qemu6, afterwards hotplug should be working once this
patch lands.

For in-place major-upgrades with machines running this doesn't really
*break* a running machine, just prevents hotplug until it was either
stop-started or a live-snapshot&rollback is performed.

If you want to go the extra mile, the running qemu version could be
queried, but I don't think we need to insist on doing this, as it just
adds code complexity for backward-compatibility across a major-version
boundary.