N+1 = 4 RedundancyWe begin by considering a small-N configuration of four hosts where the load is distributed equally to each of the hosts. For simplicity, the load distribution is assumed to be performed by some kind of load balancer with a buffer. The idea of N+1 redundancy is that the load balancer ensures all four hosts are equally utilized prior to any failover.
The idea is that none of the hosts should use more than 75% of their available capacity: the blue areas on the left side of Fig. 1. The total consumed capacity is assumed to be $4 \times 3/4 = 3$ or 300% of the total host configuration (rather than all 4 hosts or 400% capacity). Then, when any single host fails, its lost capacity is compensated by redistributing that same load across the remaining three available hosts (each running 100% busy after failover). As we shall show in the next section, this is a misconception.
The circles in Fig. 1 represent hosts and rectangles represent incoming requests buffered at the load-balancer. The blue area in the circles signifies the available capacity of a host, whereas white signifies unavailable capacity. When one of the hosts fails, its load must be redistributed across the remaining three hosts. What Fig. 1 doesn't show is the performance impact of this capacity redistribution.