High availability versus fault tolerance
The difference between fault tolerance and high availability, is this: A fault tolerant environment has no service interruption but a significantly higher cost, while a highly available environment has a minimal service interruption.
Fault tolerance relies on specialized hardware to detect a hardware fault and instantaneously switch to a redundant hardware component—whether the failed component is a processor, memory board, power supply, I/O subsystem, or storage subsystem. Although this cutover is apparently seamless and offers non-stop service, a high premium is paid in both hardware cost and performance because the redundant components do no processing. More importantly, the fault tolerant model does not address software failures, by far the most common reason for downtime.
High availability views availability not as a series of replicated physical components, but rather as a set of system-wide, shared resources that cooperate to guarantee essential services. High availability combines software with industry-standard hardware to minimize downtime by quickly restoring essential services when a system, component, or application fails. While not instantaneous, services are restored rapidly, often in less than a minute.
Many sites are willing to absorb a small amount of downtime with high availability rather than pay the much higher cost of providing fault tolerance. Additionally, in most highly available configurations, the backup processors are available for use during normal operation.
High availability systems are an excellent solution for applications that must be restored quickly and can withstand a short interruption should a failure occur. Some industries have applications so time-critical that they cannot withstand even a few seconds of downtime. Many other industries, however, can withstand small periods of time when their database is unavailable. For those industries, PowerHA® SystemMirror® can provide the necessary continuity of service without total redundancy.