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.

--kb

Suggestion for Energy Star Measurement of Blade Power Consumption

The US EPA is developing an Energy Star for Servers specification. Based on information in the latest draft of the specification, it looks like the EPA may be backing away from including servers in the first release ("Tier 1") of the server Energy Star specification. Given the increasing prevalance of blade servers in data centers, this would be unfortunate.

Ideally, there would be a standardized benchmark like SPECpower_ssj2008 that would be able to measure power consumption on a per-blade basis, but the current benchmark doesn't have provisions to handle chassis.

As an alternative, here are suggestions for how the EPA could measure power consumption for Energy Star (until a chassis-friendly industry specification is developed by an industry group like SPEC):

• Apply Energy Star to blades, not to chassis. Chassis are ineligible to meet Energy Star, but the blades that go in them can be Energy Star certified.
• Configure a chassis with the minimal amount of chassis management modules and external modules required for operation, but include all supported power supplies for a given chassis and all the fan/cooling modules typically used (don't remove redundant fans or power supplies).
• Run a sample workload on all servers to keep them minimally active. Install the same server configuration in all server slots.

Measure total power consumption to all power feeds in the chassis under two conditions and with the following calculations:

1. Condition 1: Determine power consumption P1 with all N server blade slots installed.
2. Condition 2: Remove servers so that N/2 (round up) servers are evenly distributed in the chassis; call that number N'. Determine power consumption P2 at this level.
3. P3 = P1 / N. This is the weighted average power per server blade in a full chassis.
4. P4 = P2 / N'. This is the weighted average power per server blade in a half-full chassis.
5. P5 = (P3 + P4) / 2. This is the weighted average power per server blade.

Notes:

• This accounts for chassis overhead, including fans, power supplies, management modules, and network connectivity. There is a slight penalty to blades here since rack-mount servers don't include any allocation for network switch power, but represents the minimum configuration needed to use those blades. Additionally, many vendors have low-energy networking elements (i.e., passthrough blades) that minimize this impact.
• If the chassis contains power supplies to convert input voltages to a different voltage supplied on the backplane, the power supplies used in the chassis must meet the power supply qualification requirements outlined elsewhere in the Energy Star for Servers specification.
• If a chassis contains redundant power supplies, the server blades are eligible for an allowance of 20W per redundant power supply, divided by the number of servers. For example, if a chassis has 2+2 power supplies (2 redundant power supplies and 2 minimum power supplies for a fully loaded chassis) and 10 blades, then each server would get a 4W/server allowance (2 * 20W / 10 servers).

With all the notes above, this may look to be complicated, but it's actually a fairly simple configuration that provides a close analog to how standalone rack-mount servers are tested. This could be used in the initial version ("Tier 1") of the Energy Star for Servers specification if the EPA wanted to use it.

--kb