From Corosync Cluster Engine: Designing High Availability comes a basic high availability calculation:
A = MTBF --------------- MTBF + MTTR MTBF = mean time between failures MTTR = mean time to repair A = probability system will provide service at a random time (ranging from 0 to 1)
"High Availability is achieved through the manipulation of MTBF and MTTR parameters of system design to meet availability requirements. "