Virtual Server 2005 offers Windows upon Windows

Microsoft’s Virtual Server 2005 is probably best viewed as a direct competitor to VMware’s well-entrenched GSX Server , but the degree to which Virtual Server integrates with other Microsoft server products puts it in a class of its own.

To test the product, I configured a variety of systems. The primary server bank was a pair of dual-processor Opteron rack servers, one with 4GB of RAM and one with 8GB. I focused on the Opterons because both Microsoft and VMware have adapted their products to run on Opteron with enhanced capabilities. But, similar to VMware’s products, Virtual Server 2005 is 32-bit software that doesn’t take advantage of the extended registers and math capabilities of Opteron or Intel’s EM64T extensions.

With the exception of memory, Virtual Server 2005’s system requirements are easy to meet. Windows Server 2003 is the only supported host OS, so its hardware compatibility list sets the rules. Virtual Server 2005 Standard Edition works with as many as four CPUs, whereas the Enterprise Edition supports an unlimited number of processors in a single machine.

RAM is the most significant requirement. You need what you’d ordinarily put in a server -- I consider 1GB to be the minimum for servers in the Opteron/Xeon class -- plus as much physical memory as you plan to dedicate to all of your running virtual servers combined. That adds up fast: If you only intend to run four virtual servers simultaneously, dedicating a scant 512 MB to each, you’re still looking at 3GB to 4GB of RAM.

By default, Virtual Server 2005’s virtual hard drives grow as needed; even if you allocate 20GB of disk space to a VM, it will initially occupy only as much real disk space as the installed software requires. Because storage space wasn’t an issue, however, I was able to squeeze markedly improved VM performance by using dedicated volumes on a Fibre Channel SAN.

In addition, Virtual Server’s “differencing disks” feature supports an install-once, run-many configuration. You can launch as many VMs as you please from a single disk image without interfering with the others. Virtual Server will store each machine’s data in a separate file that contains only that data which differs from the original machine’s image.

I focused most of my testing on the majority case: hosting Windows. Using each operating system’s ordinary CD-boot installation methods, I built VMs for Windows NT 4.0, Windows 2000 Server, and Windows XP. On the 8GB Opteron server, the performance of Windows burst-demand applications -- I primarily used IIS, Exchange Server, Terminal Services, and Visual Studio .Net -- was acceptable when running four virtual servers. I could push it to six by reducing the memory allocated to each VM. I was also able to balance CPU resources to favor either interactive or background sessions.