Definition

fault-tolerant

This definition is part of our Essential Guide: Server uptime and hardware failure guide
Contributor(s): Garry Kranz, Mark McCarthy

Fault-tolerant technology is a capability of a computer system, electronic system or network to deliver uninterrupted service, despite one or more of its components failing. Fault tolerance also resolves potential service interruptions related to software or logic errors. The purpose is to prevent catastrophic failure that could result from a single point of failure.

VMware vSphere 6 Fault Tolerance is a branded, continuous data availability architecture that exactly replicates a VMware virtual machine on an alternate physical host if the main host server fails.

Fault-tolerant systems are designed to compensate for multiple failures. Such systems automatically detect a failure of the computer processor unit, I/O subsystem, memory cards, motherboard, power supply or network components. The failure point is identified, and a backup component or procedure immediately takes its place with no loss of service.

To ensure fault tolerance, enterprises need to purchase an inventory of formatted computer equipment and a secondary uninterruptible power supply device. The goal is to prevent the crash of key systems and networks, focusing on issues related to uptime and downtime. Fault tolerance can be provided with software embedded in hardware, or by some combination of the two.

In a software implementation, the operating system (OS) provides an interface that allows a programmer to checkpoint critical data at predetermined points within a transaction. In a hardware implementation (for example, with Stratus and its Virtual Operating System), the programmer does not need to be aware of the fault-tolerant capabilities of the machine.

At a hardware level, fault tolerance is achieved by duplexing each hardware component. Disks are mirrored. Multiple processors are lockstepped together and their outputs are compared for correctness. When an anomaly occurs, the faulty component is determined and taken out of service, but the machine continues to function as usual.

Fault tolerance vs. high availability

Fault tolerance is closely associated with maintaining business continuity via highly available computer systems and networks. Fault-tolerant environments are defined as those that restore service instantaneously following a service outage, whereas a high-availability environment strives for five nines of operational service.

In a high-availability cluster, sets of independent servers are coupled loosely together to guarantee system-wide sharing of critical data and resources. The clusters monitor each other's health and provide fault recovery to ensure applications remain available. Conversely, a fault-tolerant cluster consists of multiple physical systems that share a single copy of a computer's OS. Software commands issued by one system are also executed on the other system.


This video from Professor Messer
describes the differences between redundancy,
fault tolerance and high availability.

The tradeoff between fault tolerance and high availability is cost. Systems with integrated fault tolerance incur a higher cost due to the inclusion of additional hardware.

What is graceful degradation?

Fault tolerance is often used synonymously with graceful degradation, although the latter is more aligned with the more holistic discipline of fault management, which aims to detect, isolate and resolve problems pre-emptively. A fault-tolerant system swaps in backup componentry to maintain high levels of system availability and performance. Graceful degradation allows a system to continue operations, albeit in a reduced state of performance.

Matching data protection and fault tolerance

Fault tolerance hinges on redundancy. Namely, information is redundantly protected via data replication or synchronous mirroring of volumes to an off-site data center. For physical redundancy, extra hardware equipment remains on standby for failover of operational systems.

Data backup is frequently combined with redundancy. Both strategies are intended as a safeguard against data loss, although backup tends to focus on point-in-time recovery, including granular recovery of a discrete data object. Redundant systems are engineered specifically for application workloads that tolerate very little downtime.

When implementing fault tolerance, enterprises should match data availability requirements to the appropriate level of data protection with redundant array of independent disks (RAID). The RAID technique ensures data is written to multiple hard disks, both to balance I/O operations and boost overall system performance.

Organizations that prioritize fault tolerance above speed and performance would be best served by RAID 1 disk mirroring or RAID 10, which combines disk mirroring and disk striping. If fault tolerance and system performance are equally important, an enterprise may find it worthwhile to spend a little extra money combining RAID 10 with RAID 6, or double-parity RAID, which tolerates the loss of two disk failures before data is lost. Aside from higher cost, the other drawback is data writes occur more slowly to the RAID set.

Aside from hardware, a fault-tolerant architecture should be coordinated with regularly scheduled backups of critical data, perhaps including a mirrored copy at a secondary or alternate location. Security needs to be part of the planning to prevent unauthorized access, and to apply antivirus tools and the most recent version of the computing system OS.

Which industries depend on system fault tolerance?

Fault tolerance refers not only to the consequence of having redundant equipment, but also to the ground-up methodology computer makers use to engineer and design their systems for reliability. Fault tolerance is a required design specification for computer equipment used in online transaction processing systems, such as airline flight control and reservations systems. Fault-tolerant systems are also widely used in sectors such as distribution and logistics, electric power plants, heavy manufacturing, industrial control systems and retailing.

This was last updated in December 2016

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