6.5.1

n
The host on which the virtual machine is running must have access to both the source and target
datastores.
n
For limits on the number of simultaneous migrations with vMotion and Storage vMotion, see “Limits
on Simultaneous Migrations,” on page 141.
CPU Compatibility and EVC
vCenter Server performs compatibility checks before it allows migration of running or suspended virtual
machines to ensure that the virtual machine is compatible with the target host.
vMotion transfers the running state of a virtual machine between underlying ESXi systems. Live migration
requires that the processors of the target host provide the same instructions to the virtual machine after
migration that the processors of the source host provided before migration. Clock speed, cache size, and
number of cores can dier between source and target processors. However, the processors must come from
the same vendor class (AMD or Intel) to be vMotion compatible.
N Do not add virtual ESXi hosts to an EVC cluster. ESXi virtual machines are not supported in EVC
clusters.
Migrations of suspended virtual machines also require that the virtual machine be able to resume execution
on the target host using equivalent instructions.
When you initiate a migration with vMotion or a migration of a suspended virtual machine, the Migrate
Virtual Machine wizard checks the destination host for compatibility. If compatibility problems prevent
migragion, the wizard displays an error message.
The CPU instruction set available to the operating system and to applications running in a virtual machine is
determined at the time that a virtual machine is powered on. This CPU feature set is based on the following
items:
n
Host CPU family and model
n
Seings in the BIOS that might disable CPU features
n
ESX/ESXi version running on the host
n
The virtual machine's compatibility seing
n
The virtual machine's guest operating system
To improve CPU compatibility between hosts of varying CPU feature sets, some host CPU features can be
hidden from the virtual machine by placing the host in an Enhanced vMotion Compatibility (EVC) cluster.
N You can hide Host CPU features from a virtual machine by applying a custom CPU compatibility
mask to the virtual machine, but this is not recommended. VMware, in partnership with CPU and hardware
vendors, is working to maintain vMotion compatibility across the widest range of processors. For additional
information, search the VMware Knowledge Base for the vMotion and CPU Compatibility FAQ.
CPU Compatibility Scenarios
vCenter Server's CPU compatibility checks compare the CPU features available on the source host, the
subset of features that the virtual machine can access, and the features available on the target host. Without
the use of EVC, any mismatch between two hosts' user-level features blocks migration, whether or not the
virtual machine itself has access to those features. A mismatch between two hosts' kernel-level features
blocks migration only when the virtual machine has access to a feature that the target host does not provide.
User-level features are non-privileged instructions used by virtual machine applications. These include
SSE3, SSSE3, SSE4.1, SSE4.2, and AES. Because they are user-level instructions that bypass the virtualization
layer, these instructions can cause application instability if mismatched after a migration with vMotion.
Chapter 11 Migrating Virtual Machines
VMware, Inc. 125