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I have a production cluster of 6 Nehalem based and two Sandy Bridge-EP based blade servers.

To enable vmotion across this cluster in vmware, I need to lower the Sandy bridge blades to nahalham using EVC, and this works fine.

My question is by doing this do I loose any capacity or functionality on the Sandy Bridge-EP based blades or is EVC simply a mask to enable vmotion across different processor types.

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Yes, you lose whatever extensions and features that became available in the 2 or 3 architecture changes since the Nehalem series was popular.

EVC brings everything down to the lowest common denominator in terms of CPU features. In practice, I only run EVC between CPUs that are one generation apart. 2 or 3 generations tends to impact the balance of the cluster in terms of significant differences in RAM capacity, performance, core count, management or even power utilization.

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It's a mask for the purposes of vMotion. It does not impair the the actual function of Sandy Bridge, it just dumbs it down so that it looks completely compatible from a Nehalem machine.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003212

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