In storage area network technology, redundancy means that essential services are implemented using duplicated components in such a way that if one component fails, an alternative component continues to provide the service. This redundancy ensures the availability of data in case a component fails.
In most RAID systems, most of the components are redundant, but the system might not be fully redundant. In other words, there might be one or two components whose individual failures would cause loss of access to data. Therefore, a fully redundant system duplicates all components and is configured to make sure that the duplicate components can be accessed in case of a failure. The manner in which the system is cabled is an essential component of creating a successfully configured redundant system.