03.Availability

Availability

A couple of parameters:

• Mean Time to Repair ($MTTR$): time between the occurrence of a fault and service recovery, also known as the mean downtime.
• Mean Time To Failures ($MTTF$): time between the recovery from one incident and the occurrence of the next incident, also known as up-time.
• Mean Time Between Failures ($MTBF$): Mean time between the occurrences of two consecutive incidents

Then we can define availability as:

$A=\frac{MTTF}{(MTTF+MTTR)}$

The probability that a component is working properly at time (actual uptime).

Reliability: means that the service is available for an agreed period without interruptions (frequency of interruptions).

$R=e-\lambda t\text{ where }\lambda =\frac{1}{MTTF}$

Note that a system could be available for 99% of time but have a lack of reliability since it continues to ‘crash’ and quickly restart.

Solve availability in practice

Simple rules:

• Components in series, so the total availability is the multiplication of all the components availability. $A_{System}=A_1\ast A_2\ast \cdots \ast A_N$
• Components in parallel: $A_{System}=(1-(1-A_1)\ast (1-A_2)\ast \cdots \ast (1-A_N))$
• The main rule to increase the availability of a system is to add in parallel another component of the one with lesser availability.
• We repeat this process until we obtain the desired availability.