One of the biggest sticking points with ESX, in my opinion, has been the VMotion compatibility and processor classes. This point has produced some rather interesting support documents both from VMware and hardware vendors. VMware has been working to alleviate this problem and demo’d the solution during VMorld again. The solution is called “Enhanced VMotion.”
The basic concept is that it takes a group of heterogenous processors, determines the lowest common denominator between the group and then sets that as the baseline for the cluster. After that point, ESX will basically mask all newer processors down to the most compatible feature set.
The problem in the past is that processors can’t hide certain features within the processor. When an application queries the processor’s CPUID, it returns some additional information about the feature sets. If an applications is running and utilizing a certain feature, then VMotions to a host without those processor features, the application will most likely blue screen, lock up or otherwise fail.
The solution which VMware has devised required intervention from its processor manufacturing partners, so the solution will only be available on certain newer processors which will allow their features to be masked. Intel’s chips with FlexMigration will support Enhanced VMotion and AMD’s Extended Migrations chips will be needed to support it.
In addition, clusters will only work with one type of processor – so in the future, we’ll have Intel-only or AMD-only clusters.  Perhaps this was always the case, but I didn’t think so.
When adding a host to an ESX cluster, all VM’s will need to be powered down. This is primarily because a running application may already be utilizing a feature which would be masked away within the cluster. So its a small inconvenience, but easy solution to get this technology to market as quickly as possible for VMware.
The Intel chips which support FlexMigration are Tigerton (Xeon 73xx), Woodcrest (Xeon 51xx), Clovertown (53xx), Dunnington and Hapertown as well as future chips.  VMware has also published a new KB article which outlines the compatible processors: KB 1003212.
