Practical ITSM Advice: Defining Availability For An IT Service

As a follow up to his presentation at the 2013 itSMF Norway conference, Stuart Rance of HP has kindly donated some practical advice for those struggling with availability.

Many IT organizations define availability for IT services using a percentage (e.g. 99.999% or “five 9s”) without any clear understanding of what the number means, or how it could be measured. This often leads to dissatisfaction, with IT reporting that they have met their goals even though the customer is not satisfied.

A simple calculation of availability is based on agreed service time (AST), and downtime (DT).

If AST is 100 hours and downtime is 2 hours then availability would be

Customers are interested in their ability to use IT Services to support business processes. Availability reports will only be meaningful if they describe things the customer cares about, for example the ability to send and receive emails, or to withdraw cash from ATMs.

Number and duration of outages

A service that should be available for 100 hours and has 98% availability has 2 hours downtime. This could be a single 2 hour incident, or many shorter incidents. The relative impact of a single long incident or many shorter incidents is different for different business processes. For example, a billing run that has to be restarted and takes 2 days to complete will be seriously impacted by each outage, but the outage duration may not be important. A web-based shopping site may not be impacted by a 2 minute outage, but after 2 hours the loss of customers could be significant. Table 1 shows some examples of how an SLA might be documented to show this varying impact.

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