6.7

n
The destination host supports the virtual machine’s feature set, plus additional kernel-level features
(such as NX or XD) not found in the virtual machine’s feature set. CPU compatibility requirements are
met, and migration with vMotion proceeds. The virtual machine retains its CPU feature set while it
remains powered on, allowing it to migrate freely back to the original host. However, if the virtual
machine is rebooted, it acquires a new feature set from the new host. This process might cause
vMotion incompatibility if you attempt to migrate the virtual machine back to the original host.
CPU Families and Feature Sets
Processors are grouped into families. Processors within a given family generally have similar feature sets.
Processor vendors define processor families. You can distinguish different processor versions within the
same family by comparing the processors’ model, stepping level, and extended features. Sometimes,
processor vendors have introduced significant architectural changes within the same processor family,
such as the SSSE3 and SSE4.1 instructions, and NX/XD CPU security features.
By default, vCenter Server identifies mismatches on features accessible to applications as incompatible
to guarantee the stability of virtual machines after migrations with vMotion.
Server hardware’s CPU specifications usually indicate whether or not the CPUs contain the features that
affect vMotion compatibility.
For more information on identifying Intel processors and their features, see Application Note 485: Intel
®
Processor Identification and the CPUID Instruction, available from Intel. For more information on
identifying AMD processors and their features, see CPUID Specification, available from AMD.
About Enhanced vMotion Compatibility
You can use the Enhanced vMotion Compatibility (EVC) feature to help ensure vMotion compatibility for
the hosts in a cluster. EVC ensures that all hosts in a cluster present the same CPU feature set to virtual
machines, even if the actual CPUs on the hosts differ. Using EVC prevents migrations with vMotion from
failing because of incompatible CPUs.
Configure EVC from the cluster settings dialog box. When you configure EVC, you configure all host
processors in the cluster to present the feature set of a baseline processor. This baseline feature set is
called the EVC mode. EVC uses AMD-V Extended Migration technology (for AMD hosts) and Intel
FlexMigration technology (for Intel hosts) to mask processor features so that hosts can present the
feature set of an earlier generation of processors. The EVC mode must be equivalent to, or a subset of,
the feature set of the host with the smallest feature set in the cluster.
EVC masks only those processor features that affect vMotion compatibility. Enabling EVC does not
prevent a virtual machine from taking advantage of faster processor speeds, increased numbers of CPU
cores, or hardware virtualization support that might be available on newer hosts.
vCenter Server and Host Management
VMware, Inc. 136