Ensuring your servers stamp as small a carbon footprint as possible on the earth and in your data center can encompass everything from making sure they are shipped in recyclable packaging to hiring an analyst who can predict the total life-cycle environmental impact.

For this test, we examined power consumption as a way to judge whether Windows Server 2008 or Linux is, in fact, the 'greener' operating system. As the price of power hits record heights, power reduction mechanisms shipping within an operating system should play a key role in you energy conservation plan.

Our tests point to Linux as the winner of the green flag by margins that topped out at 12%. But we must note that our results are full of stipulations imposed by our test bed, and as the more truthful car advertisements might warn -- your wattage may vary.

We ran multiple power consumption tests using Windows Server 2008 Enterprise Edition, Red Hat's Enterprise Linux (RHEL) 5.1 and SUSE Enterprise Linux 10 SP1 on four, popular 1U server machines, one each from Dell and IBM and two from HP. The results showed that while Windows Server 2008 drew slightly less power in a few test cases when it had its maximum power saving settings turned on, it was RHEL that did the best job of keeping the power draw in check across the board.

The variable settings allowed by both Windows and Linux – which let you toggle between having a high energy efficient server vs. a high performing one – can certainly have an impact on overall server consumption. But again, your mileage will also vary given the workloads you place on your servers and whether or not you're using popular virtual machine hypervisors to support multiple operating system instances on the same physical server (see related story).

The edge in either test category will likely not last as operating systems become more finely tuned to work in lockstep with advanced server chipsets, and as additional coding techniques that more closely tie operating systems and applications to power considerations take hold across the industry.

Part of the current "green" operating system difficulty lies in the disconnect between how an operating system and its applications can be optimized to let the underlying system quiet itself down to a lower power-consuming state while at the same time not sacrificing the ability to react to servicing application (and therefore system and user) needs.

In our testing, we found that the CPU 'throttle-back' mechanism – the main technique for how an operating system can aid in reducing a server's energy draw -- requires new firmware and updated drivers that specifically support that feature. Only the IBM x3350 and the HP DL-360 G5 arrived ready for optimal power conservation. The HP DL-160 and Dell 1950 servers required several updates throughout our six-week test period to accommodate the CPU throttling features of Windows 2008 and Linux.

We truly know from the trenches that it really isn't easy getting your servers to be green.

CPU conservation

No matter the operating system, Windows or Linux, the leading form of power conservation comes from throttling back the CPU to let the server rest during quiet activity times. Spinning down hard disks to a quieter state is the other major power-saving setting available to Windows servers.

Even though Linux desktop distrobutions can use the Advanced Configuration and Power Interface (ACPI) specification, which is designed for laptops, rather than servers, that feature was not implemented by Red Hat or SUSE for the servers we tested.

Some chipsets are designed for throttle-back, while others (especially older ones, predating 2007) always work at full pace and power full time. Only in the past three years have processors shipped from Intel, Advanced Micro Devices, Via Technology, and others in the x86 family have been specifically designed to cycle between fast (and higher power consuming) and slow (power savings) states.

The systems used in our testing ship with Intel Xeon multi-core CPUs, which can support throttle-back (manifested by slowing down the CPU clock so that the power strobes through the CPU more slowly against the slowed clock); but as we stated earlier the server's BIOS and firmware must be sufficiently upgraded to correctly supported this. The IBM x3350 and HP DL-160 house a single quad-core CPU (of different models, see How we did it), while the Dell 1950 and HP DL360G5 each housed two quad-core CPUs for a total of eight cores.