GutoVeronezi commented on pull request #4878:
URL: https://github.com/apache/cloudstack/pull/4878#issuecomment-865990220


   @slavkap thanks for testing this feature.
   
   QEMU demands the DOM to be prepared to support CPU scaling and hotplug 
memory. This preparing is made on VM deploy. ACS verifies if the service 
offering is dynamic, VM has dynamic scale enabled and global setting 
`enable.dynamic.scale.vm` as `true`:
   
   
https://github.com/apache/cloudstack/blob/607dc234adb16043bd8a2851b56da3e129e90139/server/src/main/java/com/cloud/hypervisor/KVMGuru.java#L180-L186
   
   If they all are enabled and there is a difference between current and max 
(memory/CPU), the DOM is prepared with the following code:
   
https://github.com/apache/cloudstack/blob/607dc234adb16043bd8a2851b56da3e129e90139/plugins/hypervisors/kvm/src/main/java/com/cloud/hypervisor/kvm/resource/LibvirtVMDef.java#L268-L281
   
   Therefore, if you deploy the VM with the global setting as `false`, it will 
not prepare the DOM and will result in an error, as 
`org.libvirt.LibvirtException: unsupported configuration: cannot use/hotplug a 
memory device when domain 'maxMemory' is not defined`, when trying to scale it.
   
   


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