Earlier this year, I wrote a three-part series of columns about Exchange Server virtualization, starting with "Exchange Server Virtualization: Microsoft's Support". In the second part, "Exchange Server Virtualization: Hyper-V Possibilities," I talked about what I expected Microsoft to support for Exchange 2007 virtualization with Windows Server 2008’s Hyper-V technology—once the company got around to giving us any official information on the matter. On August 19, Microsoft finally ended the waiting and announced its support policies for running virtual Exchange 2007 SP1.

So now we know Microsoft officially supports Exchange Server 2007 SP1 running under Hyper-V. The support statement, "Microsoft Support Policies and Recommendations for Exchange Servers in Hardware Virtualization Environments," features a wealth of interesting information. First off, Exchange 2007 SP1 is supported for virtualization only when running on a virtualized instance of Server 2008; because Virtual Server 2005 doesn’t (and won’t) support 64-bit guest virtual machines (VMs), Exchange 2007 is only officially supported on Hyper-V.

It’s possible that Microsoft might support VMware’s server virtualization solutions in the future; VMware will have to meet the requirements of the Server Virtualization Validation Program (SVVP). Chris Wolf, a senior analyst for the Burton Group, reported that VMware had joined the SVVP, but we’ll have to wait until Microsoft or VMware announce that VMware’s products have passed SVVP testing before Exchange will be officially supported on those products.

Microsoft's Exchange virtualization support statement says that the Exchange Unified Messaging role isn’t supported for virtualization, but the Mailbox, Hub Transport, and Client Access roles are. That’s good news because it means that you can use virtualization along with cluster continuous replication (CCR) and standby continuous replication (SCR), although Microsoft cautions against combining CCR, SCR, and single copy clusters (SCC) with high availability solutions provided by the hypervisor, such as VMware VMotion or Hyper-V’s quick migration. The built-in Exchange solutions can perform checks, such as server heartbeat detection, that the hypervisor-based solutions can't.

From a storage perspective, official virtualization support doesn’t change that much. Dynamic virtual disks aren’t supported, nor are differencing disks, though both of these remain enormously useful for test and demo environments. Traditional SCSI and iSCSI are both supported. The combination of Hyper-V and iSCSI is an intriguing design that I plan to write about in the future.

Perhaps the most intriguing sentence in the document is this one: “Guest virtual machines on some hypervisors will exhibit processor core scalability trends that are different from processor scalability trends exhibited by physical machines.” That statement seems to me to be a shorthand way of saying that the resource use exhibited by a given Exchange configuration under load might be very different between a virtualized environment and a similar environment running directly on hardware. This concept argues strongly in favor of performing stress and load testing on virtualized environments before you put them into production. Thankfully, that testing is much easier to do with virtualized environments than with real ones!