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On the other hand, a highly available system such as one served by a load balancer
allows minimal downtime and related interruption in service without total

redundancy when a failure occurs. A system with some critical parts mirrored and
other, smaller components duplicated has a hybrid strategy.

In an organizational setting, there are several important concerns when creating
high availability and fault tolerant systems:

Cost. Fault tolerant strategies can be expensive, because they demand the
continuous maintenance and operation of redundant components. High
availability is usually part of a larger system, one of the benefits of a load
balancing solution, for example.

Downtime. The greatest difference between a fault-tolerant system and a highly
available system is downtime, in that a highly available system has some minimal
permitted level of service interruption. In contrast, a fault-tolerant system should
work continuously with no downtime even when a component fails. Even a
system with the five nines standard for high availability will experience
approximately 5 minutes of downtime annually.

Scope. High availability systems tend to share resources designed to minimize
downtime and co-manage failures. Fault tolerant systems require more, including
software or hardware that can detect failures and change to redundant components
instantly, and reliable power supply backups.

Certain systems may require a fault-tolerant design, which is why fault tolerance
is important as a basic matter. On the other hand, high availability is enough for
others. The right business continuity strategy may include both fault tolerance

and high availability, intended to maintain critical functions throughout both
minor failures and major disasters.

Fault Tolerance Architecture

There is more than one way to create a fault-tolerant server platform and thus
prevent data loss and eliminate unplanned downtime. Fault tolerance in computer
architecture simply reflects the decisions administrators and engineers use to

ensure a system persists even after a failure. This is why there are various types
of fault tolerance tools to consider.

At the drive controller level, a redundant array of inexpensive disks (RAID) is a
common fault tolerance strategy that can be implemented. Other facility level
forms of fault tolerance exist, including cold, hot, warm, and mirror sites.