When moving VMs from an ESX 3.5 cluster to an ESX 4 “vSphere” cluster in VMWare you will have to look at upgrading the VMWare Tools for sure. However, you also have the option to upgrade the virtual hardware of a VM. The virtual hardware upgrade offers some performance benefits, but not every VM will be able to take advantage of it. It is also not mandatory to upgrade the virtual hardware, however if you do want to upgrade the virtual hardware, you need to upgrade the VMWare tools to the latest version first AND you need to go through a full reboot cycle first. Upgrading the virtual hardware of a VM requires to shut it down as the option is only available in a powered off state of a VM.
So far I have upgraded the virtual hardware on several VMs in my environment and the overall experience has been good (knock on wood). In one instance I had missed to update the VMWare Tools to the latest version and upon the attempt to upgrade the virtual hardware my vCenter politely asked me if I really wanted to proceed. I did not and opted to update the VMWare Tools first.
What can you expect after the upgrade? You can expect a plug and play fest so to speak. It’s like ripping out the motherboard and all the other components from a physical server and to replace them with newer ones. So far I upgraded several operating systems including Windows 2000, Windows 2003, and Windows 2008. No issues other than the usual “plug and play spiel” inside Windows.
I still have to do some performance testing, but overall things seem to be speedier.