In a move that surprised no one, on August 19th, Microsoft announced support for Exchange Server 2007 when running under Hyper-V on Windows Server 2008. Go team!

Further, in a move that surprised almost everyone, Microsoft also announced their Windows Server Virtualization Validation Program (SVVP) in which they will work with other vendors to certify those virtualization solutions for hosting Microsoft applications and servers. As I write this article on September 3, VMware announced today that they had been certified in the SVVP. Even more, go team!

These rather dramatic changes allow for Exchange Server 2007 service pack 1, every server role excepting Unified Messaging, to be run in a virtual server instance. There are some caveats behind this, which we will discuss shortly.

However, in a somewhat surprising turn of events, Exchange Server 2003 is NOT supported for Hyper-V or VMware. Exchange Server 2003 is supported for virtualization - but only with Virtual Server 2005 R2 (or any later release of Virtual Server). Exchange Server 2003 must also have service pack 2 (or later) installed to be supported in a virtualized environment, and they may not be installed in a Windows fail-over cluster.

Running Exchange Server 2003 in Virtual Server has some significant limitations. These include the fact that Virtual Server only supports a single processor, so only a single core of a single processor is available for the virtualized copy of Exchange Server. Also, you must use the VHD file format for storing the Exchange store and log files, which results in an additional level of indirection (and therefore inefficiency) in disk input-output operations. iSCSI is not a supported option. You should test carefully before deploying Exchange Server 2003 in production in a virtualized environment.

Returning to Exchange Server 2007, the set of requirements are relaxed significantly, but still represent restrictions over a hardware based installation:

• Storage must be VHD less than 2 TB, iSCSI, or pass-through disk
• Dynamically expanding disks and differencing disks are not supported
• You can use CCR or SCC as a high-availability (HA) solution between virtual instances, but the physical virtual servers may not be clustered
• Exchange is not compatible with hypervisor-based HA solutions (Quick Motion from Hyper-V or Live Migration from VMware, etc.) when Exchange is part of a clustered set of virtual instances
• Snapshots of an Exchange guest are not supported for backup
• Etc.

The major comments from the Exchange Product Team are, unsurprisingly, warnings about performance. Virtualized environments will always (probably!) perform less well than native hardware environments. When you are virtualizing an application, such as Exchange, which requires significant memory, processor, and disk resources - you absolutely need to ensure that adequate resources are dedicated to the virtual instance.

In general, it is not recommended to place multiple high-resource-requirement virtualized application environments on a single physical server, unless the performance under load has been validated and found acceptable.

Even with the restrictions, the fact that Microsoft will now officially support Exchange Server in a virtualized environment, including VMware’s virtualized environment, is a huge step forward to the Microsoft messaging ecosystem. It may be something for you to evaluate for your environment.