Enhanced Vmotion

I was just reading up a little more on Enhanced Vmotion as was impressed that we no longer have to worry about CPU architecture and found the following useful information:

Enhanced VMotion Compatibility

Because new features are constantly incorporated into new CPUs, you are likely to encounter incompatibilities and therefore face problems while attempting migration with VMotion. Such CPU incompatibilities limit VMotion to a certain range of CPUs. VMware has investigated the issues and concluded that there is no software‐only solution to this problem. To minimize exacerbation of the compatibility problem with time, VMware has worked with CPU vendors to facilitate live migration of virtual machines across different CPU revisions.

VMware Enhanced VMotion Compatibility (EVC)—available in VMware Infrastructure 3 beginning with version 3.5 Update 2—facilitates VMotion between different CPU generations, taking advantage of Intel Flex Migration and AMD‐V Extended Migration technologies. When enabled for a cluster, EVC ensures that all CPUs within the cluster are VMotion compatible. CPUs starting with Intel 45nm Core 2 (Penryn) and AMD Second Generation Opteron (revision E or F) incorporate FlexMigration and Extended Migration technologies, respectively.

The EVC feature allows the virtualization layer to mask or hide certain features by modifying the semantics of the CPUID instruction and hides certain CPUID feature bits, even from nonprivileged code. For example, with support from hardware, the virtualization layer modifies the semantics of the CPUID instruction to mask or hide the SSE4.1 feature from any code (privileged or nonprivileged) to make CPUs differing in this feature compatible for VMotion.

Specifics on CPU compatibility with the Enhanced VMotion Compatibility feature are available in the Basic System Administration guide for each ESX release starting with version 3.5 Update 2.

EVC utilizes hardware support to modify the semantics of the CPUID instruction only. It does not disable the feature itself. For example, if an attempt to disable SSE4.1 is made by applying the appropriate masks to a CPU that has these features, this feature bit indicates SSE4.1 is not available to the guest or the application, but the feature and the SSE4.1 instructions themselves (such as PTESE and PMULLD) are still available for use. This implies applications that do not use the CPUID instruction to determine the list of supported features, but use try‐catch undefined instructions (#UD) instead, can still detect the existence of this feature.

Therefore, for EVC to be useful, application developers must adhere to recommended guidelines on feature detection. CPU vendors recommend that software programmers query CPUID prior to using special instructions and features available on their CPUs. If this guideline is followed by programmers, EVC is a reliable mechanism for live migration of x86 virtual machines across varied hardware. Thus, you can use EVC to enable an entire cluster to use the same set of basic features, allowing migration with VMotion across any two nodes in the cluster. VirtualCenter can also set up new hardware add‐ons to the cluster and apply these masks.

Virtualization is still a relatively new technological innovation, and the area of live migration of virtual machines is still in its infancy. VMware VMotion is an extremely useful and critical feature in data centers to ensure business continuity, resource optimization, and a host of other benefits. VirtualCenter performs numerous compatibility checks before allowing migration with VMotion. Some stringent CPU compatibility checks for VMotion are necessary for proper functioning of a virtual machine after migration. Though users can override these checks and complete VMotion by applying appropriate masks, the virtual machines and applications may not function properly if they rely on features particular to the underlying hardware. The x86
CPU vendors did not initially envision live migration of virtual machines and design CPUs to support it.

VMware has worked with CPU vendors to support such features, taking advantage of Intel Flex Migration and AMD‐V Extended Migration technologies. With Enhanced VMotion Compatibility, VMware Infrastructure works in conjunction with hardware to support live migration of virtual machines in a wider range of environments. Effective use of enhanced VMotion also depends on using application software that follows recommended guidelines to support CPU feature detection.

4 thoughts on “Enhanced Vmotion

  1. Denis Baturin

    I meant, how to disable migration at all…
    For example i`d like to disable live migration for virtual machines with MSCS… Only offline…

  2. Virtu-Al

    You can do this from within the cluster settings and VMware DRS / Virtual machine options, set to disabled.

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