What is Fault Tolerant System ?

Fault Tolerant System -:

Fault tolerance is the property that enables a system to continue operating properly in the event of the failure of (or one or more faults within) some of its components. If its operating quality decreases at all, the decrease is proportional to the severity of the failure, as compared to a naively designed system in which even a small failure can cause total breakdown. Fault tolerance is particularly sought after in high-availability or life-critical systems. The ability of maintaining functionality when portions of a system break down is referred to as graceful degradation.^[1]

A fault-tolerant design enables a system to continue its intended operation, possibly at a reduced level, rather than failing completely, when some part of the system fails.^[2] The term is most commonly used to describe computer systems designed to continue more or less fully operational with, perhaps, a reduction in throughput or an increase in response time in the event of some partial failure. That is, the system as a whole is not stopped due to problems either in the hardware or thesoftware. An example in another field is a motor vehicle designed so it will continue to be drivable if one of the tires is punctured. A structure is able to retain its integrity in the presence of damage due to causes such as fatigue, corrosion, manufacturing flaws, or impact.

Within the scope of an individual system, fault tolerance can be achieved by anticipating exceptional conditions and building the system to cope with them, and, in general, aiming for self-stabilization so that the system converges towards an error-free state. However, if the consequences of a system failure are catastrophic, or the cost of making it sufficiently reliable is very high, a better solution may be to use some form of duplication. In any case, if the consequence of a system failure is so catastrophic, the system must be able to use reversion to fall back to a safe mode. This is similar to roll-back recovery but can be a human action if humans are present in the loop.

Abstract

In the past few years, all manner of storage systems,

ranging fromdisk array systems to distributed and widearea

systems, have started to grapple with the reality

of tolerating multiple simultaneous failures of storage

nodes. Unlike the single failure case, which is optimally

handled with RAID Level-5 parity, the multiple failure

case is more difficult because optimal general purpose

strategies are not yet known.

Erasure Coding is the field of research that deals with

these strategies, and this field has blossomed in recent

years. Despite this research, the decades-old strategy of

Reed-Solomon coding remains the only space-optimal

(MDS) code for all but the smallest storage systems.

The best performing implementations of Reed-Solomon

coding employ a variant called Cauchy Reed-Solomon

coding, developed in the mid 1990’s [BKK+95].

In this paper, we present an improvement to Cauchy

Reed-Solomon coding that is based on optimizing the

Cauchy distribution matrix. We detail an algorithm

for generating good matrices and then evaluate the

performance of encoding using all manners of Reed-

Solomon coding, plus the best MDS codes from the literature.

The improvements over the original Cauchy

Reed-Solomon codes are as much as 83% in realistic

scenarios, and average roughly 10% over all cases that

we tested.

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