System availability

System availability

System Availability

• The percentage of time that the system can perform its intended function

System Availability = 1 – System Downtime


Downtime per year

Availability	Nines	Downtime
90%	1	36.5	days/year
99%	2	3.65	DAys/year
99.9%	3	8.78	hours/year
99.99%	4	52	minutes/year
99.999%	5	5	minutes/year

System Downtime

Many events causes system downtime:

• HW fault
• Software fault
• Vandalism
• Extreme conditions (fire, flooding etc)
• Power outage
• IP network failure
• Planned system maintenance

System Downtime = ∑ P * S * MTTR

P	= Probability of event taken place
S	= Severity of event
	= Percentage of service affected by fault
MTTR	= Mean Time To Repair
	= mean time to detect fault + mean time to fix fault