I know nothing about WebSphere other than it exists; as you may have gathered, I try to avoid IBM products. I was talking to an engineer yesterday and he mentioned that WebSphere was a product he was never able to run successfully as a virtual machine on ESX 3.X. The memory was constantly being paged and the hypervisor couldn’t keep up.
That got me to thinking: would W2008 R2 Hyper-V’s SLAT (second level address translation) feature help with this? I suspect it probably would make virtualising this application more feasible. SLAT leverages AMD RVI and Intel EPT to remove the hypervisor from the role of mapping physical memory to virtual machine memory. The CPU is able to do it more efficiently than software can.
It’s something that might be worth testing in a lab if a WebSphere server has appeared on your radar as a candidate for conversion. Just make sure you are using SLAT capable CPU’s in your host servers.
ESX 4.0 has something similar to SLAT so I guess it is probably worth trying there too if that is your hardware virtualisation platform.
As usual, check with the vendor for virtualisation support and recommendations. Then balance the risks and decide for yourself.