Service Availability - MTTR, MTTD, MTTF, MTBF

Availability of a Website or service(A) = Uptime / (Uptime + Downtime)

Availability of a website or service is measured on four important parameters

• MTTD(Mean Time To Diagonise) = The average time it takes to diagonise the problem
• MTTR(Mean Time To Repair) = The average time it takes to fix a problem
• MTTF(Mean Time To Failure) = The average time there is a correct behaviour 
• MTBF(Mean Time Between Failures) = The average time between failure of service(average time between two failures)

MTBF = MTTD + MTTR + MTTF

Availability  = MTTF / MTBF

Service Availability expressed in terms of 9s

90% (one 9) Availability = 36 days,12 hours downtime/year

99% (two 9s) Availability = 87 hours, 36 minutes downtime/yr

99.9% (three 9s) Availability = 8 hours,45 minutes, 36 seconds downtime/yr

99.99% (Four 9s) Availability = 52 minutes,33 seconds downtime/yr

99.999% (Five 9s) Availability = 5 minutes,15 seconds downtime / yr

99.9999% (Six 9s) Availability = 31 seconds downtime/yr

If we say we provide, 99.9% uptime availability per month, then permissible downtime is calculated as follows

30 days = 720 hours = 43200 minutes

99.9% of 43200 minutes = 43156.8 minutes

43200 minutes - 43156.8 minutes = 43.2 minutes

So the service can go down without penalty for 43.2 minutes per month