What is N+1 and 2N redundancy?

N means exactly what you need with zero backup. N+1 means one spare. 2N means everything is doubled. The notation tells you how many things can fail before you lose power or cooling. N means the exact number needed with no backups - if one unit fails, the system fails. N+1 means one additional backup unit exists: in a system needing 4 cooling units, N+1 provides 5. If any single unit fails, the remaining 4 handle the full load. 2N means the entire system is duplicated: two complete, independent sets of infrastructure. If an entire power feed fails, the second one carries the full load. 2N+1 adds one extra unit on top of the duplicated system. Higher redundancy directly correlates with higher uptime and higher cost. N+1 is standard for Tier III facilities; 2N is standard for Tier IV.