Saturday, October 15, 2011

Tips: How and when to use Processor Compatibility Mode to Migrate Virtual Machines

By default in a cluster, all the nodes will be same. However, the hardware migration is fairly straightforward. We can there are no concerns about differences in hardware. As Hyper-V can take advantage of the processor capabilities in the newest Intel and AMD processors for improving the overall speed and efficiency of the VMs on the physical host, the default should be used whatever processor features on their original host. Both the live migration and Quick Migration will work fine with the identical processors. If a cluster comprises nodes with different processors, the capabilities of the processors will be different as a migration occurs with a running machine. Moreover, this may cause a failure when the VM tries to run after migrating to a different processor. Applications use the x86 CPUID processor instruction to determine the processor type and processor features.

When Processor Compatibility Mode is used, Hyper-V hides the processor features by intercepting a VM’s CPUID instruction and clearing the returned bits that correspond to the hidden features.

Make sure that you can use the Processor Compatibility Mode where VMs will migrate from one Hyper-V-enabled processor type to another within the same vendor processor family. Though the names of the processors give the impression, the Processor Compatibility Mode doesn’t enable the migrations between AMD- and Intel-based hosts. Processor Compatibility Mode is not needed for VM moves that involve a stop and restart of the VM. This includes unplanned failovers and manual VM moves between hosts.

You can see the settings dialog box of Hyper-V Manager to enable the VM Processor Compatibility. If you wish to enable the Processor Compatibility Mode on existing VMs, it is required to shut down the VM and change the Processor settings for VM.


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