We setup all of our virtual machines in our new data center to run with a single virtual processor. The thought behind this was to start with the minimum and add additional resources as needed after observing our VM’s under load.

Well the time has come to add a second virtual processor to three of our VM’s, all of which are running Server 2008 data center edition. In the past, adding a second processor (or even enabling hyper threading) could cause issues due to an incorrect HAL (hardware abstraction layer) being chosen when you first install the OS. I thought I had read that this is not an issue under Vista and Server 2008, but couldn’t find anything to confirm (other then the lack of people reporting problems).

Since we are using virtual machines (VMWare ESX Server), it’s easy enough to take a snapshot and revert if something goes terribly wrong. However, the snapshots were NOT needed, as the servers booted up without issue after adding the second virtual processor.

Something I might want to consider next time would be to disable ESX’s dynamic resource allocation routine before shutting down three VM’s.