Free Cooling: the Server Side of the Storyby Johan De Gelas on February 11, 2014 7:00 AM EST
How do these higher temperatures affect the fans?
It is clear that the fan speed algorithm takes more than just the CPU temperature and inlet temperature into account. There's definitely an ability to detect when a low power CPU with low tCase is used. As a result the fans are spinning faster with the 2650L than with the 2697 v2. That also means that the server has more headroom for the Xeon E5-2697 v2 than we first assumed based on the CPU temperature results. At higher inlet temperatures, the fans can still go a bit faster if necessary on the 2697, as the maximum fan RPM is 7000.
The big question of course is how all this affects the power bill. It's no use saving on cooling if your server simply consumes a lot more power due to increased fan speeds (and potentially down time when replacing fans more frequently).
The difference in power consumed is not large between the three inlet temperatures. To make our measurements clear, we standarized on the measurements at 20°C as the baseline and created the following table:
|Xeon E5-2697 v2||Xeon E5-2650L|
|CPU load||Inlet 20°C||Inlet 35°C||Inlet 40°C||Inlet 20°C||Inlet 35°C||Inlet 40°C|
As the fans work quite a bit harder to keep the 2650L below the low Tcase, they need a lot more power. We notice a 9% increase in power when the inlet temperature doubles. The increase is smaller with the Xeon E5, only 7%.
The most interesting conclusion is that raising the inlet temperature from 20 to 35°C results in almost no increase in power consumption (3-5%) on the server side, while the savings on cooling and ventilation can be substantial, around 40% or more.