With the new Intel and AMD processors having multiple cores, today’s Hyper-V hosts can easily have 24 or more cores. Just buy a mainstream server with dual hexa-core Hyper Threaded CPU’s and your performance monitor will look like a bar chart; the bars are actually your CPU utilization graph..
I’ve seen multiple situations where a multi-core server is occupied by one or two virtual servers, each using the maximum of 4 cores, hence leaving a lot of cores unused.
Windows Server 2008 R2 Hyper-V officially supports 4 cores per VM. Windows 8 Server will give us a lot of new features and upgrades, and will also support more than 4 cores per VM. They might even add multiple core support to the next SP for Server 2008 R2.
There is a way to add more cores to your VM by editing your VM’s XML file:
Step 1: Shut down your virtual machine
Step 2: Locate and open the XML configuration file of your VM. You can find the (default) location in the Hyper-V settings in the actions pane.
Step 3: Edit the <count type=”integer”> setting and change the number in the desired amount of cores. I heard this can go up to (and maybe beyond) 32 logical cores.
Step 4: Save the file
Step 5: Start the server and check in the task manager if your server has the correct amount of cores.
There seems to be issues with Microsoft System Center Virtual Machine manager with displaying the server properties, I haven’t tested this in a live system center environment to see for myself.
I wouldn’t recommend doing this in a production environment, since it is not officially supported my Microsoft, and can potentially have some unknown effects on your Hyper-V host and other VM’s running on the same machine.
More info on this subject can be found in this thread.