Clustering Servers-

“erver clustering reduces the number of backup or standby servers needed by a system, improves availability, and uses hardware more efficiently. Traditionally, data centers required one backup server for each primary working server. That no longer is necessary. N+1 server clustering technology allows for only one backup server per cluster of servers. For example, instead of needing six passive servers to back up six active servers, with N+1 clustering you need only one passive server. “

https://www.energystar.gov/products/low_carbon_it_campaign/12_ways_save_energy_data_center/consolidation_lightly_utilized_servers