Derive component availability from MTBF and MTTR, then the availability an N+M redundant group delivers.
A single component's availability follows directly from how often it fails and how long repairs take. Adding spares multiplies the failure probabilities together, so each additional redundant unit removes roughly one more order of magnitude of downtime — provided failures really are independent. It converts an SLA target into a concrete answer about whether N+1 is enough or N+2 is required, and it exposes MTTR as usually the cheapest lever to pull.
Network Redundancy
component availability = MTBF ÷ (MTBF + MTTR); system unavailability = (1 − availability)^(spares + 1), where spares = installed − required.
component availability = MTBF ÷ (MTBF + MTTR); system unavailability = (1 − availability)^(spares + 1), where spares = installed − required. A single component's availability follows directly from how often it fails and how long repairs take. Adding spares multiplies the failure probabilities together, so each additional redundant unit removes roughly one more order of magnitude of downtime — provided failures really are independent.
It converts an SLA target into a concrete answer about whether N+1 is enough or N+2 is required, and it exposes MTTR as usually the cheapest lever to pull.
This calculator takes 4 inputs: Components installed, Components required to carry load, Mean time between failures, Mean time to repair. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Availability depends on the ratio of repair time to uptime, and MTTR is the term you control. Halving MTTR from four hours to two doubles availability just as effectively as doubling MTBF, and it costs a spares cupboard rather than new hardware.