Christophe, In our benchmark testing on ESX 4 and ESX 5 we've proven that there is no significant difference in CPU or Memory requirements between a VMware virtualized instance and a native instance. In fact during some tests the Virtual instance performed tasks faster than the native instance. The new Windchill 10 sizing guides are now available (for 10.0 and 10.1). Our current ESX 5 testing also shows that we get some great scalability out of their 8 and 16 core capabilities now too. So for the very large enterprise it looks very positive that we can use very large VM's now with confidence. On ESX 4 we recognized that the larger you made the VM the less linear scalability you received. Therefore two 4vCPU instances had more capacity available to them than we could get out of one 8vCPU isntance.
The only thing that we truly recommend, as does VMware, is that for at least in the production VM's you use VMware's Memory Reservation option (it's not enabled on VM's by default) to earmark the physical memory required to support your JVM's in that host. If your host has 16Gb of RAM and 12GB of that is earmarked for JVM's in your configuration we would at least recommend a reservation of not less than 12GB so that you do not encounter the overhead associated with memory virtualization. That's actually a bad thing for Java to use the virtualized layers for memory and it is directly attributed to most of the performance issues we recognize at customers where virtualization is used.
CPU over allocation is encouraged so long as the ESX host CPU threshold stays at or below 60 to 70%. Once you reach those thresholds you just need to remain satisfied with the response times you are getting. Generally we consider a ESX host fully utilized when it gets to the 60 to 70% CPU threshold.
