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An N-to-1 failover configuration reduces the cost of hardware redundancy and still provides a potential, dedicated spare. In an asymmetric configuration there is no performance penalty. There there are no issues with multiple applications running on the same system; however, the drawback is the 100 percent redundancy cost at the server level.
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N-to-1 configuration
An N-to-1 configuration is based on the concept that multiple, simultaneous server failures are unlikely; therefore, a single redundant server can protect multiple active servers. When a server fails, its applications move to the redundant server. For example, in a 4-to-1 configuration, one server can protect four servers. This configuration reduces redundancy cost at the server level from 100 percent to 25 percent. In this configuration, a dedicated, redundant server is cabled to all storage and acts as a spare when a failure occurs.
The problem with this design is the issue of failback. When the failed server is repaired, all services that are hosted on the server must be failed back to the server. The failback action frees the spare server and restores redundancy to the cluster.
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N-to-1 failover requiring failback
Most shortcomings of early N-to-1 cluster configurations were caused by the limitations of storage architecture. Typically, it was impossible to connect more than two hosts to a storage array without complex cabling schemes and their inherent reliability problems, or resorting to expensive arrays with multiple controller ports.