There are two major versions of Windows Server in widespread use: Windows Server 2003 and Windows Server 2008, which has now reached R2 and brings some major enhancements. In addition, there are a dozen or more versions of Linux, as well as other flavors of Unix, such as Solaris, OSX and BeOS, not to mention a variety of virtualization platforms that can run multiple OSes on a single piece of hardware (with enough CPU power and RAM).
One of the major dividing lines at the moment is 32-bit versus 64-bit. Windows 2003 had a 64-bit version, though drivers weren't always available for much of the hardware you may have wanted to run. Six years later, Windows 2008 R2 is 64-bit only -- it doesn't support 32-bit processors. Nearly everything else currently available has a 32-bit and a 64-bit version, so you need to know which you want to run. Support for 64-bit allows for much more memory than the 4 GB supported by 32-bit OSes, and better utilization of the latest processors, which results in faster, more stable operation.
Virtually all server hardware shipped in the last year is 64-bit, so if you're buying new hardware, there's no reason not to run 64-bit versions unless you have specific applications or hardware that doesn't have 64-bit support yet. Most new hardware will have 64-bit drivers -- at least for Windows 2008. But if you have specific hardware that you need to run, such as Fibre Channel or Infiniband HBAs, 10 Gigabit Ethernet, or specialized products like multi-port Ethernet cards, point-of-sale, or serial port cards, you'll need to check carefully to ensure that 64-bit drivers are available before you proceed.
Likewise, if you want to upgrade your server OS on existing hardware, you'll need to carefully determine whether the hardware will support a 64-bit OS. This can be difficult to determine solely from model numbers, as a definitive list is often not available. Microsoft does have a pre-qualification tool that you can run to determine whether specific hardware will run Windows Server 2008, but for 64-bit versions of Linux, your best bet is to try the newsgroups and vendor sites.
In some cases, new OSes may have more stringent requirements than a 64-bit processor. Virtualization products such as VMware vSphere and Microsoft's HyperV require virtualization support on the CPU -- Intel VT-x or AMD-V. Determining whether a specific processor supports this can be difficult, as some models in the same line have support while others don't. In the case of specialized functions such as LiveMigration in HyperV, there can be other hardware requirements as well, such as specific driver support from storage vendors for Microsoft Cluster Shared Volumes.
In short, if you're installing a new 64-bit OS on new hardware, you probably won't have problems unless you need to install specialized hardware or support legacy applications. With older hardware, you may discover that getting 64-bit applications running is still not a slam dunk.
ABOUT THE AUTHOR: Logan Harbaugh is a freelance reviewer, network systems analyst and consultant, specializing in reviews of network hardware and software, including network operating systems, clustering, load balancing, network-attached storage and storage area networks, traffic simulation, network management and server hardware.
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This was first published in October 2009