VMs need to be powered off if they are running CPU feature sets from a newer processor generation than what you are trying to set in EVC. E.g. If the VM is running (and was powered on) on a Haswell generation processor but you are trying to set the EVC to Sandy Bridge.
What generation CPUs do you have in your cluster and what EVC mode are you trying to set to?
If the validation is successful then you can enable it without any issue. However, if you reboot any VM before setting the EVC, the rebooted VM will start using the full feature set of the physical CPU and if you are setting the EVC to a older generation than the physical CPU, then you will need to power off that VM. Same applies if a VM has migrated between same generation CPU.
I am trying to set it to Nehalem.
I know one devices has rebooted on the "older" CPU, but I don't believe any device has rebooted on the newer CPU.
So as long as I haven't rebooted any machines since moving it from an EVC Compatible cluster, I can enable EVC Compatibility on the cluster with no issues?
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Yes as long as no VM has rebooted/powered on on a newer generation host, you will be able to enable EVC. Its always a good idea to have EVC enabled even though you have hosts with the same generation processor.
Thanks so much for your help and assurance, I turned it on and was able to migrate the machines through the cluster.