What is a PDU in an intelligent, energy-aware data center?

Next-generation PDUs share more information with data centers, which can improve understanding of data center conditions.

Next-generation PDUs provide granular energy and environmental monitoring and management across the existing data

center LAN.

At its simplest form, a power distribution unit (PDU) organizes electrical wiring and controls power to individual servers -- like a power strip in your home. But over time, the question "what is a PDU" has evolved. Today's PDUs provide a wealth of data about power use, efficiency and environmental conditions in the data center, all networked across the LAN and managed through a Web browser or command-line interface (CLI).

Power-monitoring features

A rack-based PDU's primary role is to control the distribution of power to servers, storage units, network switches and other equipment in each rack. This puts PDUs in the unique position to make energy-related measurements and calculations that can detail how much energy is used and how efficiently.

A PDU that measures input voltage and current to each circuit can calculate power used in watts or kilowatts, and the energy used in kilowatt-hours. It can also calculate the total power capacity used, letting the data center operator know when they're close to needing additional PDUs.

The granularity of these measurements varies with PDU products. Some PDUs may only calculate at the total PDU level -- for example, the total power and energy of everything connected to the PDU. You can therefore determine the total energy characteristics of the rack, but not of its individual devices.

For better granularity, look for PDUs that monitor down to the socket level. IT staff can more easily locate power-hungry, less energy-efficient systems that are prime candidates for replacement during the next technology refresh cycle.

PDUs with power-up sequencing features can stagger the activation of each outlet, avoiding dangerous inrushes of current when the rack recovers from a catastrophic power loss. For example, a PDU may be configured to power up storage systems and networking equipment first, and then systematically power up individual servers.

Use the power data obtained from PDUs to help calculate broader energy metrics such as power usage effectiveness (PUE). For example, the sum of power used in each rack can be compared to the total power used in the building to form the basis for a PUE calculation.

Environmental monitoring features

Data centers throw off a lot of heat. New PDUs integrate multiple temperature and humidity/water sensors. Heat and humidity are critical factors for rack equipment, especially for data centers using chilled water or free air for server cooling.

Intelligent PDUs track environmental factors and produce timely alerts. In addition, administrators can compare environmental data to energy data and correlate the actual effect that server power use and conservation measures have on temperature behaviors within the rack.

Some PDUs include dry contact sensors to detect physical conditions, such as rack door openings. This physical monitoring can help prevent unauthorized tampering by personnel, an oft-overlooked data center security vulnerability.

PDU access and networking features

Monitoring important characteristics isn't much help unless administrators can access the data easily. Look for PDUs that provide versatile LAN access via a Web server or CLI.

A built-in Web server and firmware allow administrators to track the PDU's current status and review measured data over time. The Web interface enables programmable options, such as setting alert thresholds and options for Simple Network Management Protocol traps or other email-alerting alternatives. PDUs with Web server support typically support Secure Sockets Layer and Secure Shell security features and may support network features such as Dynamic Host Configuration Protocol, Active Directory and Lightweight Directory Access Protocol. The latest PDUs support IPv4 and IPv6 network protocols for device addressing.

PDUs with CLI support use more traditional serial interfaces such as Telnet or RS-232. While the interface is simpler than Web server access, a technician must directly access the PDU to connect. A CLI also requires a good grasp of the PDU's command set -- which can be extensive. Considering the current workload of IT administrators, CLI and Telnet management is less attractive than a Web-based portal.

What a rackmount PDU can be has evolved from simple power switch to an intelligent measurement and monitoring tool. PDUs' detailed power and environmental information can help data centers improve energy use and efficiency, as well as thermal behavior within each rack.

This was first published in December 2013

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