Tuesday, March 31, 2009

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.


  • 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.


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