A. The decision about whether to purchase the Standard, Enterprise, or Datacenter edition of Windows Server 2008 R2 should depend on two major factors: the number of virtual machines (VMs) you intend to run and your high availability requirements.

High availability is only available with the Enterprise and Datacenter SKUs, so if you want clusters and features like Live Migration, you have to use Enterprise or Datacenter edition.

The next factor is the number of VMs. Standard Edition supports one physical OS and one virtual OS (VM), Enterprise supports one physical OS and four virtual OSs, and Datacenter supports one physical OS and an unlimited number of virtual OSs. Note that Standard and Enterprise are purchased on a per-server basis while Datacenter is purchased on a per-processor basis, and at least two processors (sockets) must be licensed on each server with Datacenter.

You can assign multiple licenses to a single physical server. For example, I could purchase two copies of Enterprise Edition and assign them to a single physical server, which would allow me to run eight VMs. I could also buy eight copies of Standard Edition or just two of Datacenter (I need two because two is the minimum number purchasable with Datacenter—two sockets).

Generally, the following is a good guideline for the most cost effective SKU to buy, but remember to consider future growth.

  • Standard Edition is most cost efficient for one to three VMs per server. Note that if you run three VMs, you'll need to buy three copies of Standard Edition.
  • Enterprise Edition is most cost efficient from four VMs on a server up to four VMs per processor. For example, if I have a dual processor box and want to run eight VMs, I could buy two copies of Enterprise edition.
  • Datacenter Edition is most cost efficient for more than four VMs per processor, because you can run an unlimited number of VMs per processor and license each processor. While Datacenter is more expensive than Enterprise when running four VMs per processor, you have more scalability and support for future growth, so you could, potentially, adopt Datacenter over Enterprise when you consider future requirements. Remember that you have to license all processors in the server.

Microsoft offeres an online calculator that can help you choose the right SKU to buy.

Remember that if you're using the maximum number of virtual instance rights, you can't run any workloads other than Hyper-V in the parent partition. If you run additional workloads in the parent partition (which isn't recommended), you lose one of your virtual instance rights. So if you're running Enterprise Edition and you're also running a file and print server on the Hyper-V host, you can only run three virtual OS instances.

When you purchase a SKU of Windows Server, you have the right to run that version and any lower version in your virtual environments. For example, if I buy Datacenter Edition, I can run Datacenter, Enterprise, or Standard in my VMs. If I buy Enterprise edition, I can run Enterprise or Standard in my VMs.

Finally, remember you can't move these virtual OS environment rights between servers. If you want to run four VMs on each server and have the ability to live migrate them to another server (which would mean it would run eight VMs), you need two licenses of Enterprise on the target server (or Datacenter). This is why when you're using Live Migration and clusters, it's normally advised to purchase Datacenter.