In case you didn’t think it through (like I didn’t), you can’t vMotion guests from one type of CPU to another. And when I say type, I don’t just mean AMD to Intel, I mean different models and generations. For example, you can’t go from Family 6, Model 46 (2Eh) = Nehalem-EX to Family 6, Model 47 (2Fh) = Westmere-EX/E7… at least not live. It makes sense. However, there is one exception: EVC.

CPU instructions are different and to change that mid-stream on a running programs who have loaded libraries based on the expected CPU instructions, just won’t work. If you can’t vMotion, why even cluster? VMware will let you put them in the same cluster, but won’t let you vMotion. OVM won’t even let you put them in the same cluster. I haven’t tried with Xen or RHEV yet.

EVC is a way to dumb down members of a VMware cluster to make members of the cluster to who have different cpus, be able to vmotion (in certain circumstances). You get to pick the oldest CPU type, and VMware will hide the newer CPU instructions to the guests via CPU bitmap {I have to look up exactly how that works}. Perhaps this could help inspire other vendors to help their customers.

To take advantage of this, “Edit Settings” on your cluster and click on “VMware EVC”, click on “Change EVC mode” and hopefully you can figure the rest out.

VMware KB on EVC