1. Examples of N+1 redundancy
     N+1 redundancy    1+1 redundancy    2+1 or 3+1 redundancy  

2. Applications

3. See also

4. References

It is also possible to have N+1 redundancy with active-active components, in such cases the backup component will remain active in the operation even if all other components are fully functional, however the system will be able to perform in the event that one component is faulted and recover from a single component failure.

Examples of N+1 redundancy

N+1 redundancy

• Connecting devices (server etc.) in dual switch SAN (Storage area network) fabrics employ a discrete path to each switch. Only one path is active at any given time, resiliency is provided by the availability of an additional path if the active path becomes unavailable.
• Data centre power generators that activate when the normal power source is unavailable.

1+1 redundancy

• Dual active power supplies in a server.
• Mirrored hard drives within a server/PC system.

2+1 or 3+1 redundancy

2+1 redundancy or 3+1 redundancy is common on power systems for blade servers where a relatively small number of highly rated PSUs efficiently power a greater number of blades. An example is a server chassis that has three power supplies; the system may be set to 2+1 redundancy so that the blades can enjoy the power of two PSUs and have one available to give redundancy if one fails. It is also common to mix live (hot) redundancy where PSUs are online, and cold standby redundancy where they are offline until needed. The reason for this, in the case of PSUs, is that a common failure mode is component's end-of-life failure, and if PSUs are equally used, then they are highly likely to fail within a short space of time of each other when toward the end of service life.

Applications

Redundant systems are often used in data centers to ensure that computer systems continue without service interruption. Other common implementations include aerospace, where redundant systems are used to improve safety and integrity of spacecraft, electric power systems and automobiles, where the emergency brake is available in a car as a redundant component in case of failure of the main brake systems.

See also

• High availability
• High-availability cluster
• RAID

References

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