While researching my talk for TechEd tomorrow, I came across a good Windows Server 2008 Performance Tuning whitepaper. There was a small section on power management which provided a good overview of how it works in Windows Server 2008. Essentially, where Windows Server 2003 didn't have a power savings plan implemented by default, Windows Server 2008 has the 'Balanced' plan which saves about 10% energy on baseline Windows Server 2003.
"High performance and low power usage are often conflicting goals, but by carefully selecting server components you can determine the correct balance between them. Table 3 contains guidelines for power characteristics and capabilities of server hardware components.
Table 3. Server Hardware Power Savings Recommendations
|
Component |
Recommendation |
|
Processors |
Higher frequencies in a specific processor family cause increased power consumption when the processors are under heavy load. Also, processor families usually include low-power versions. Newer generations of processors expose more power states for the Windows power management algorithms, which enables better power management at all levels of performance. |
|
Memory (RAM) |
Memory consumes an increasing part of system power. Many factors affect the power consumption of a memory "stick" such as memory technology, error correction code (ECC), frequency, capacity, density, and number of ranks. Therefore, it is best to compare expected power consumption ratings before purchasing large quantities of memory. Low-power ("green") memory is now available, but a performance or monetary trade-off must be considered. If paging is required, then the power cost of the paging disks should also be considered. |
|
Disks |
Higher RPM means increased power consumption. Also, new 2.5-inch drives consume less than half the power of older 3.5-inch drives. More information about the power cost for different RAID configurations is found in "Performance Tuning for Storage Subsystem" later in this guide. |
|
Network and storage adapters |
Some adapters decrease power consumption during idle periods. This becomes a more important consideration for 10-Gb networking and high-bandwidth storage links. |
The default power plan for Windows Server 2008 is Balanced, which tries to keep performance high while it saves power whenever possible. The other predefined plans are Low Power and High Performance, both of which are heavily weighted to different goals. But server BIOS settings can prevent Windows from accomplishing any of these goals, so make sure that you check whether power management by the operating system or by the hardware is a BIOS option. Windows Server performance lab tests show that Windows power management works very well when it is compared to hardware-managed power management on enterprise servers, so the operating system-managed setting is preferred. However, the most important guideline is to make sure that the BIOS settings on a specific server are well understood so that the administrator knows if the Windows power setting controls (including the High Performance plan) are actually usable."
It also provides tips on choosing RAID systems if you want to balance cost, capacity and power; as well as how power access affects RAID.
You can download the entire document HERE.
