Applying Open Compute Rack and Power Specs in Your Data Center

Kevin Gero is Strategic Account Manager for Bel Power Solutions.

The Open Compute Project (OCP) has the goal of accelerating the commercialization of hardware designs that enable greater data center efficiency and flexibility. This is, of course, attractive to all; but OCP tends to be driven by the needs of large greenfield data center developers. In the past, this created challenges in applying the specifications to smaller, brownfield data centers. Fortunately, as the specifications have evolved, they have become more flexible and more applicable to a wider range of data centers.

Rack Size and Configuration

For example, in its initial Rack and Power specification, the OCP redefined the dimensions of the IT equipment rack from the traditional 19-inch to 21-inch wide racks. Adding two inches to the width of the rack creates more room for cable runs, simplifies server deployment and service, and improves airflow.

All of that makes sense, but if you have a data center full of 19-inch racks, you’re probably reluctant to begin adding 21-inch racks. The OCP community later developed an EIA-compliant 19-inch rack specification to accommodate those applications.  

Both the 19-inch and 21-inch specification unbundle the traditional server configuration within the rack. Servers, storage and power are separated on shelves within the rack (see Figure). This allows servers to be upgraded more cost-effectively when new processors are introduced. It also provides greater flexibility in configuring racks with required equipment.

One limitation some designers initially faced when considering the viability of the OCP Rack and Power specification is rack density. The OCP rack supports 13.2 kW of IT equipment through the use of two power shelves which each consist of three power supplies in an N + 1 configuration.

However, power shelves have adapted and are now available with six 3 kW power supplies in an N + 1 configuration. This effectively doubles the power capacity of an OCP rack, opening the specification to higher density racks. Due to the design of modern power supplies, which have a maximally flat efficiency curve across the load range, these higher density shelves can be deployed without compromising efficiency, even if the initial capacity of the rack is well below the design specification.

As a result, designers and operators seeking to leverage the benefits of the OCP specifications can now choose between 19-inch and 21-inch racks, with a full complement of racks, servers, storage and power products available for each.

The Question of Voltage

Another issue designers face is determining the input voltage to the rack and output voltage from the power shelf to the servers.

To increase efficiency, the OCP specification brings 277/480 V power (230/400 V in Europe) directly from the transformer to the rack, eliminating the stepdown to 208 V. This can create a 2.5 percent efficiency improvement compared to 208 V distribution without making significant changes to the way power is delivered to the rack.

Output voltages can also vary. The initial OCP specification provides for 12 V output from the power shelf, which is what most server hardware is designed to accept. But when Google joined the OCP in 2015, they developed an OCP specification for 48 V output from the power shelf.

Delivering 48 V to the server reduces the voltage drop across the busbar and connectors by a factor of 16 (assuming same conductor cross section), increasing the overall efficiency compared to 12 V. The bigger advantage of 48 V, however, is that it provides a path to the future for racks with much higher power requirements. It’s still too early to determine whether the OCP 48 V specification will drive broad commercialization of 48 V servers, but the initial signs are promising.  If you’re seeking to achieve the highest possible efficiency and are willing to live with the limits of the current market for 48 V servers, this could be a viable option. The current generation of power shelves can be configured to accommodate 208 V AC, 277 V AC or 380 V DC input voltages and 12 V or 48 output voltages.

Making the Move to OCP

As the OCP specifications continue to mature, they are being applied to a wider range of environments and requirements. With a mission of improving the efficiency and flexibility of data center hardware, the OCP is helping advance new approaches to data center design that have broad benefits within the industry. While not everybody is well positioned to adopt the OCP specifications, there is flexibility within the current specifications and the equipment that supports them; and data center designers and developers should, at minimum, stay abreast of the developments within the OCP and how they are shaping the future of data center hardware.

Opinions expressed in the article above do not necessarily reflect the opinions of Data Center Knowledge and Informa.

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