Datacenter Edition builds on the feature set offered by Enterprise Edition and eliminates a few features that aren’t considered appropriate for a large enterprise data center. For example, Datacenter does not support the Internet Connection Firewall or Internet Connection Sharing, two features designed to make Windows an Internet gateway for network clients. Both features are, however, supported by Standard Edition and Enterprise Edition. Datacenter Edition adds support for up to 64GB of server RAM and up to 32-way processor support. These features require a specialized Hardware Abstraction Layer (HAL), which is provided by the server hardware manufacturer. Datacenter Edition’s Cluster Service supports eight-way clusters, enabling you to build more complex failover clusters for mission-critical applications.

Although Datacenter Edition does provide a "bigger and better" product over Enterprise Edition and Standard Edition, that’s not really the point of Datacenter Edition. The real point of Datacenter Edition is much more important and is almost philosophical in nature: You can only buy Datacenter in conjunction with an approved server, directly from an approved server vendor. To receive product support for Datacenter Edition, you must contact the server vendor, not Microsoft, and you cannot make any hardware changes to the server without prior approval from the server vendor.

This philosophy is at the heart of Microsoft’s Datacenter program, which is designed to provide you with a server that can remain up and running 99.999% of the time—the magical "five nines" reliability number that enterprises demand. In case you’re wondering, that’s slightly less than nine hours of downtime per year.

Everything about the Datacenter program is focused on reliability:

• All hardware included in a Datacenter-approved server must meet rigid Microsoft standards and pass a battery of compatibility and reliability tests.

• All device drivers must be certified and digitally signed by Microsoft.

• Customers cannot make any unauthorized changes to the server hardware. This includes every aspect of the hardware. For example, if you purchase a quad-processor Datacenter system, you can’t upgrade it to eight processors unless that’s also a certified, supported configuration from your vendor.

As we mentioned earlier, Datacenter Edition is sold only through server vendors and is sold preinstalled only on certified server hardware. You’ll find that the hardware on which Datacenter Edition is offered is usually the highest of the high end: multiprocessor computers with copious amounts of RAM, redundant network adapters and power supplies, and so forth. Datacenter Edition computers are almost always clustered for higher availability and generally run an enterprise’s most mission-critical applications.

Microsoft takes more care with updates for Datacenter Edition, too. Operating system service packs and hot fixes for Datacenter Edition generally lag behind such releases for other editions of Windows Server 2003 because Microsoft and its server vendor partners rigorously test all fixes for the operating system to ensure nothing will interfere with Datacenter Edition’s 99.999% reliability record.

Likely applications for Datacenter Edition include any high-volume, mission-critical use, such as the following:

• Massive file and print servers

• Large, non-partitionable databases

• Consolidated servers that each assume the functionality of multiple lesser servers

• Other enterprise applications that simply cannot be unavailable

Expect to pay for Datacenter Edition’s reliability. Although Microsoft doesn’t publish pricing for Datacenter Edition are established by the server vendors who resell the operating system—the operating system itself, not to mention the high-end server hardware on which it runs, commands a premium price. If you’re interested in finding out exactly how much, contact your local Hewlett-Packard or IBM sales representative. Tell him you’re interested in purchasing a Datacenter Edition computer and watch his ears perk up! In fact, the sheer expense of Datacenter Edition and the associated server hardware makes it the least-deployed version of Windows. Many administrators might go their entire career without working on a Datacenter Edition, even in fairly large enterprises.