Thursday, April 16, 2015

Data Center Design Consideration: Redundancy

So far we have discussed about the Cooling and Space Planning issues for data center design. We all note that data centers are very special purpose-built facilities providing critical and uninterrupted services. In this section we are going to explore one of the key considerations - Redundancy.

Redundancy entails providing system beyond the minimum capacity to ensure these systems continue to operate even if part of the system fails. Under the TIA-942 Standards, data centers are classified into different tiers based on their availability and redundant designs. What exactly are these designs?

For more information of Tier Levels: N, N+1, 2N, 2(N+1), please visit our previous articles:

N Design

An N design means the number of components and paths is exactly what is required to meet the data center requirements. There is neither spare capacity nor any standby unit. Basically, there is no redundancy and the system will totally fail when a component or path fails.

N+1 Design

An N+1 design requires the number of components installed to exceed the requirements by one. In this case, when one of the components fails, the standby unit will take over and the system would continue to operate.

2N Design

As the name mentions, a 2N design has 2 independent working systems supporting the data center. In the event a primary system fails, a secondary system would take over and continue the uninterrupted operations.

2(N+1) Design

A 2(N+1) design is an extension of the 2N design. Both the primary and secondary systems are equipped with a single group of (N+1) facilities, e.g., each system has an extra component. This design is more resilient than a 2N design and would be able to withstand a concurrent path and component failure.

It is important to understand that in 2N or 2(N+1) systems, the secondary system should be located away from the primary system which minimizes the primary and secondary system from being damaged by any incidents, such as fire, floods, etc.

The following examples illustrate the design of a UPS (Uninterruptible Power Supply) system supplying 200kVA of power to a data center:

Figure 1: Various redundant design for a UPS system providing 200kVA of power

In an N Design, one UPS of 200kVA is installed. This design would meet the 200kVA requirements of the data center. Normal operation is disrupted when the UPS fail.

An additional UPS is installed in an N+1 Design. This brings the number of UPS system to 2 which allows for normal operation if one UPS fails. E.g., the remaining UPS would still able to support the minimum 200kVA power. However, the system can be still failure if the path (feeder cable) linking the UPS to the equipment is damaged.

A 2N Design offers protection for both path and component failures. This requires 2 UPS systems (Primary and Secondary) each with 1 unit of 200kVA UPS. Should a UPS fails to start or a cable feeder is damaged, the Secondary System would take over and continue to supply the required load.

The 2(N+1) Design is the most resilient of the 4 designs discussed. 2 systems with 2 units of 200kVA UPS each are installed. This design would continue to provide the necessary 200kVA power even if there is a simultaneous component and path failure. It should be noted, however, no design is 100% fail-proof and this design would still fail in the event of multiple failures.

The pictures below show the resiliency of various systems under different failure conditions:

Figure 2: Different redundant design under different failure conditions

The Blogger

Strategic Media Asia (SMA) is one of the CPD Course Providers of the Chartered Institution of Building Services Engineers (CIBSE).

SMA, a critical infrastructure training and event organizer based in Hong Kong, provides an interactive environment and opportunities for members of IDC industry and engineers to exchange professional views and experience on critical infrastructure and E&M facilities.

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