Cofiguring EVC on a vSphere cluster

Enhanced vMotion Compatibility (EVC) is a feature of vSphere clusters which allows vMotion between hosts with CPUs of different generations (e.g. a host with an Intel Xeon E5520 CPU and a host with an Intel Xeon E5-2630 CPU). If you have hosts with different generation CPUs, properly setting EVC is a prerequisite for adding these hosts to the same cluster. Setting EVC on a cluster basically forces CPUs of all hosts in the cluster to use a common baseline - a set of instructions which is compatible with all CPUs in the cluster. 

EVC is a vSphere feature that can be enabled regardless of vSphere license edition you're using. It can be enabled on a cluster by right clicking on a cluster in your inventory -> Edit Settings -> VMware EVC -> Change EVC Mode.

After choosing Change EVC mode, you will be presented with options for Enabling EVC for AMD hosts or Enabling EVC for Intel hosts. Select your CPU of choice and head to this VMware KB article - Enhanced vMotion Compatibility (EVC) processor support.

This KB article is the best possible EVC reference in which you can find which EVC cluster baselines are supported by different CPU models. All you need to do is find the (not required but recommendedhighest) baseline which is supported by all of your host CPUs and configure EVC to work in this mode. Although you can choose any baseline supported by all CPUs, it is recommended to choose the highest baseline, because of reasons described at the end of the post. For example, if you need to create a cluster with the previously mentioned E5520 hosts and E5-2630 hosts, you would set "Intel Nehalem Gen." as your EVC mode. The EVC menu can also be of help, because when you choose an EVC mode it will tell you whether the hosts which are already a part of the cluster support that EVC mode.

There are a few things which you need to have in mind when configuring EVC in an existing cluster: 

• when EVC is disabled, this is equal to all host CPUs working with the highest supported EVC baseline
• if you have a cluster with older hosts (e.g. with E5520 CPUs) with EVC disabled, and you need to add new host(s) to the cluster (e.g. with E5-2630 CPUs), you need to first enable EVC in the proper mode and then add new hosts; this can be done without any disruption - setting a CPU to work with it's highest supported baseline (in this example "Intel Nehalem") can be done with VMs running on the hosts
• if you need to add an older host (e.g. a server left after virtualizing a service it used to run) to a cluster with EVC disabled and newer hosts which are running VMs, you will be configuring hosts to work in a lower EVC baseline than they are currently; in order to do this, all hosts in the cluster have to be in maintenance mode, which means that all VMs running on them need to be powered off (or migrated to a different cluster, if possible)