Fault Tree Analysis
Fault Tree Analysis (FTA) is a technique that can be used to determine the chain of events that causes a disruption to IT services. FTA, in conjunction with calculation methods, can offer detailed models of availability. This can be used to assess the availability improvement that can be achieved by individual technology component design options. Using FTA:
FTA makes a representation of a chain of events using Boolean notation. Figure 4.19 gives an example of a fault tree.
Figure 4.19 Example Fault Tree Analysis
Essentially FTA distinguishes the following events:
These events can be combined using logic operators, i.e.:
This is the basic FTA technique. This technique can also be refined, but complex FTA and the mathematical evaluation of fault trees are beyond the scope of this publication.
To assess if new components within a design can match the stated requirements, it is important that the testing regime instigated ensures that the availability expected can be delivered. Simulation, modelling or load testing tools to generate the expected user demand for the new IT service should be seriously considered to ensure components continue to operate under anticipated volume and stress conditions.
Modelling tools are also required to forecast availability and to assess the impact of changes to the IT infrastructure. Inputs to the modelling process include descriptive data of the component reliability, maintainability and serviceability. A spreadsheet package to perform calculations is usually sufficient. If more detailed and accurate data is required, a more complex modelling tool may need to be developed or acquired. The lack of readily available availability modelling tools in the marketplace may require such a tool to be developed and maintained ‘in-house’, but this is a very expensive and time-consuming activity that should only be considered where the investment can be justified. Unless there is a clearly perceived benefit from such a development and the ongoing maintenance costs, the use of existing tools and spreadsheets should be sufficient. However, some System Management tools do provide modelling capability and can provide useful information on trending and forecasting availability needs.