Redpaper - IBM Redbooks

These kinds of systems will operate with an amount of planned and
unplanned outages. There are two significant aspects in this definition:
– The system should survive a single failure but a second failure may result
in a loss of service.
– The detection of a fault and the triggering of an action to recover from it
should be automatic, that is, require no manual intervention.
Figure 1-1 illustrates the relationship between the components of systems
availability.
Continuous Availability
Concurrency
Redundancy
Systems Management
Reliable, Robust and Resilient Technologies
High Availability + Continuous Operation
Figure 1-1 High availability + Continuous Operation = Continuous Availability
There are times with the term fault tolerance is used mistakenly when the terms
high availability or continuous availability are meant. Fault tolerance describes
systems which, in the event of a failure, can substitute a replacement component
for the failed component in a matter of a few milliseconds. This kind of
achievement is supported by components that have redundant sub-components,
error checking and correction for data, retry capabilities for basic operations,
alternate path for I/O requests, and so forth. However, there may also be a single
point of failure which, despite the fault tolerance, can cause a component to fail.
Similarly, if one important component in a system is not fault-tolerant, then the
system is not fault-tolerant even though all other components are.
Specifically, HA refers to a specific level of service that provides availability in the
event of a single, non-catastrophic component failure. Transaction capacity may
Chapter 1. Introduction and technical overview 3