Sunday, March 7, 2010

Energy Star for Server Should Require Right-sized Power Supplies

The EPA is currently developing Tier 2 (second-phase) requirements for servers with the Energy Star ratings. Although the EPA is probably right to back away from the non-standard definition they were developing to capture "net power loss" for servers, they should still look at adjusting server power supply efficiencies when power supplies are oversized.

Some vendors may ship servers that only draw 200W at 100% utilization with a power supply that can provide 1200W. Traditional ways of evaluating power supplies measure those power supplies across their full rated capacity. If a power supply is sized appropriately, this makes sense. However, a power supply that is too much higher than the system will see in real life should be de-rated.

For examaple, a 2000W power supply might have good efficiency at 50% and 100% of load, but power efficiency tends to drop off at lower load levels, particularly those below 25% of maximum load. Take that same 2000W power supply and put it in a server drawing a maximum of 200W, and the power supply wout always be operating below 10% load. The normal power supply rating levels are of little value if the realistic power draw is much lower than the rated power draw.

To be fair, the tested configurations of servers don't always represent the highest possible loading: adding extra memory, additional hard drives, and extra PCI Express cards can increase a servers power draw. But having no upper limit leaves too much wiggle room and jeopardizes the integrity of the Energy Star rating method.

One possible solution to work around this is as follows:
  1. Measure the server power consumption under an acceptable benchmark such as SPECpower_ssj2008. Record 2x the maximum power draw (i.e., at 100% load in the benchmark).
  2. Look at the rated output power for the power supply or power supplies needed to operate the server in that configuration [ignore redundant power supplies used for reliability purposes]. Record the sum of the power of all the non-redundant power supply output power ratings.
  3. If the answer in Step 2 is less than or equal to the value from Step 1, no adjustment is needed. Skip Steps 4 and 5.
  4. If the answer in Step 2 is more than the value in Step 1, plot the efficiency rating of the non-redundant power supplies. Extrapolate the efficiency of the power supply (power supplies) at the value recorded in Step 1. Extrapolate the efficiency at 50% and 25% of the value shown in Step 1. Do the same for any other power supply levels normally required, but rate them as a ratio of the value shown in Step 1.
  5. Evaluate the efficiency of the system based on the load levels and efficiency determined in Step 4 above.

This adjustment would correct ratings for power supplies oversized for the systems they're being tested with. This will incent server vendors to right-size power supplies to better match the real power range of the systems they're being rated for.