While we've already noted that RHEL conserved more power than Windows Server 2008 in most cases, we'd be remiss if we did not also mention that RHEL drew less power than its Linux brethren in all quiescent tests. The spread for that difference was a low of .5 watts less on the IBM server when the systems were tuned for top performance to a high of over 5 watts less on the HP DL-160G5 server when the systems were in power savings mode.

During the active tests, Windows Server 2008 running in power-savings mode on the Dell box used over 7 per cent more power than the Linux average on the same hardware. But on the IBM and HP DL-360G5 server, Windows Server 2008 pretty much ran on par with the lowest Linux consumer in those tests.

When running in the high performance mode in the active test, Windows Server 2008 used as much as 11 per cent more wattage than the average Linux power draw on the same hardware. That said, Windows Server 2008 had the best power consumption rating in this test run on the HP DL-160G5 server, spending on average about 6.5 watts less than Linux.

The server hardware impact

Server makers responded to our requests to be part of the test bed with several kinds of CPU and disk configurations all housed within 1U hardware casings. Overall, the savings wasn't startling between the highest and lowest number for a server in terms of watts consumed in any test.

There's little doubt that advertised power savings numbers can be achieved, but servers will need to be tuned for both the operating system, as well as the power-savings applications' ability to control which cores are either used or put to rest in a load-balancing situation. Rather than push configurations for core optimizations (we couldn't find settings to do this for the applications tested), we let the operating systems take care of the details. The details, it turns out, was that all of the cores in all of the tests saw at least some activity both in quiescence but also in application use. SMP kernels were used for all tests.

IBM's x3550 was the leanest and greenest, both in terms of CPU 'horsepower' but also power consumed. In quiescent tests, there was less than a 2-watt difference among any of the three operating systems tested in either performance or savings mode. In the active tests, the power draws stayed within that 2-watt range with the exception of when Windows Server 2008 drew 87.8 watts, compared with SUSE's 79.6 watts and RHEL's 78.3 watts in our active test when the systems were tuned from performance.

The dual-quad core Dell 1950 sucked more power overall, but with more cores, it also delivers more computing power. In the quiescent tests, the test where settings were tuned for performance, Red Hat defied logic and used slightly more power than it did in the power savings mode, but otherwise, the results followed as logic and settings expected.

The HP DL-160 didn't show dramatic behavior changes in settings in the quiescent tests, and made Windows 2008 Server a winner in the active, performance-modes test where it seemed to give its best performance.