Consideration regarding enabling Cluster EVC Mode when Target CPU Generation is the same with the physical host CPU Generation

Products

• VMware vCenter Server

Issue/Introduction

• Four hosts in a cluster use Cascade Lake CPUs.
• One host with a Sapphire Lake CPU is going to be added to the cluster 
• The customer is not willing to power-off the runnig VMs.

Environment

VMware vCenter Server

VMware ESXi 

Cause

The document below does not directly mention whether we need to power-off the running VMs if the target EVC Mode(CPU Generation) is the same as the host's physical CPU Generation.https://techdocs.broadcom.com/us/en/vmware-cis/vsphere/vsphere/7-0/vcenter-and-host-management-7-0.html

 

EVC Mode	Virtual Machine Power Action
Raise the EVC mode to a CPU baseline with more features.	Running virtual machines can remain powered on. New EVC mode features are not available to the virtual machines until they are powered off and powered back on again. A full power cycling is required. Rebooting the guest operating system or suspending and resuming the virtual machine is not sufficient.
Lower the EVC mode to a CPU baseline with fewer features.	Power off virtual machines if they are powered on and running at a higher EVC Mode than the one you intend to enable.

 

 

The document below mentions that all powered on virtual machine with no EVC mode or with an EVC mode that is greater than the EVC mode that you plan to enable on the cluster must powered off. https://techdocs.broadcom.com/us/en/vmware-cis/vsphere/vsphere/7-0/vcenter-and-host-management-7-0.html

Resolution

If the running VMs need to load the new CPU instruction set and features, a full power cycling is required.

 

For example, if we want to set the cluster EVC Mode with Cascade Lake, the VMs on the four hosts don't need to be powered off.

The VMs on the host with a Sapphire Lake CPU need to be powered off so they can adjust to the EVC mode with a lower CPU baseline featuring fewer capabilities.

 

Therefore, if the target EVC Mode(ex. Cascade Lake CPU) is the same as the host's physical CPU Generation(ex. Cascade Lake CPU),

powering off the VMs is not required.

 

However, there are some exceptional situations, such as the following:

Some Running VMs might have hidden CPU features, which is leading the host not to be able to mask and set lower its CPU instruction sets. In this situation, depending on the VM Virtual Hardware Version, you might not be able to enable the cluster EVC mode even if the target CPU Generation is the same as the physical CPU Generation.

It is very hard to check the applied instruction sets on all the running VMs, so it will be easier to verify that there are no compatibility errors before enabling the cluster EVC Mode or powering off the VMs.