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Information about High_Availability

Published on March 8, 2009

Author: dsgiri

Source: authorstream.com

High-Availability : High-Availability DS Giri Dharmendra.giri@gmail.com http://SliceData.BlogSpot.com Introduction to Reliability : Introduction to Reliability No matter what service is being performed by a computer system, users must have confidence in how the system operates in order to be able to use it under good conditions. The term "reliability" characterises how trustworthy a computer system is. A failure is when a service does not function properly, i.e. a state of operation that is abnormal or, more precisely, not in accordance with specifications. From the user's point of view, a service has two statuses: appropriate service, i.e. in accordance with expectations inappropriate service, i.e. not in accordance with expectations A failure is attributable to an error, i.e. a local dysfunction. Not all errors lead to service failure. There are several ways to limit service failure: Error prevention, which consists of avoiding errors by anticipating them Fault tolerance, the goal of which is to provide a service that is in accordance with specification despite errors by introducing redundancy Error elimination, aiming to reduce the number of errors through corrective actions Error prediction, by anticipating errors and their impact on service Introduction to High-Availability : Introduction to High-Availability "High-availability" is all the measures that aim to guarantee service availability, i.e. ensure around-the-clock operation of a service. The term "availability" refers to the probability that a service is operating properly at a given time. The term "reliability", which is also sometimes used, refers to the probability that a system is operating normally over a given period of time. This is called "continuity of service". Availability is most often expressed by the availability rate (a percentage), which is measured by dividing the time the service is available by the total time. Availability is most often expressed by the availability rate (a percentage), which is measured by dividing the time the service is available by the total time. Risk Evaluation : Risk Evaluation Indeed, the failure of a computer system can cause losses in productivity and money and even material and human losses in certain critical cases. Thus, it is necessary to evaluate the risks tied to the dysfunction (failure) of one of the components of a computer system and anticipate the means and measures to be used to avoid the incidents or to reestablish service in an acceptable amount of time. As everybody knows, there are numerous ways in which a network computer system can fail. The causes of failures can be broken down as follows: Physical causes (these can be natural or criminal in nature): Natural disaster (flood, earthquake, fire) Environment (bad weather, humidity, temperature) Material failure Network failure Power cut Risk Evaluation : Risk Evaluation Human causes (these can be intentional or accidental): Design error (software bug, poor network provisioning) Human causes (these can be intentional or accidental): Design error (software bug, poor network provisioning) Operational causes (these are linked to system status at a given moment): Software bug 2.Software failure All of these risks can have different causes such as the following: Intentional maliciousness Fault Tolerance : Fault Tolerance Since it is impossible to totally prevent breakdowns, one solution consists in setting up redundancy mechanisms by duplicating critical resources. The ability of a system to operate despite the failure of one of its components is called fault tolerance. When one of the resources breaks down, the other resources take over in order to give system administrators the time to find a solution to the problem. This is called "Fail-Over Service" (FOS). Ideally, in the case of material failures, the faulty material elements should be hot swappable, i.e. capable of being extracted and replaced without service interruption. Backup : Backup Setting up a redundant architecture ensures that system data will be available but does not protect the data against user-introduced errors or against natural disasters such as fires, floods or even earthquakes. Therefore it is necessary to set up backup mechanisms (ideally remote) in order to guarantee data perenniality. Moreover, a backup mechanism can also be used for archival storage, i.e. saving data in a state that corresponds to a given date. Types of Backup : Types of Backup No matter what damage takes place, the implemented backup mechanism absolutely must be designed to ensure the perenniality and recovery of all of a company's critical data without disrupting the running of the computer system. Therefore, choosing a backup mechanism must be the subject of a backup strategy that defines the data to be backed up, the backup frequency and method, and a disaster recovery plan that indicates the steps necessary for reestablishing normal functioning in case of a problem. Backups are generally divided into the following categories. Full backup Differential backup Incremental backup Delta backup Logging Types of Backup : Types of Backup Load Balancing : Load Balancing Load balancing consists in distributing a task to a pool of machines or peripherals in order to: even out network traffic, i.e. distribute the overall burden to different machines ensure equipment availability by sending data only to those machines that are able to handle it or to those machines that have the best response time This type of mechanism relies on a load balancer, whose job it is to distribute the work between different machines. There are many ways to implement load balancing: With a layer 4 switch With a server that uses a Round Robin algorithm Clusters : Clusters The Concept of Clusters A "cluster" is an architecture made up of several computers that form nodes, where each node is able to operate independently. There are two main types of clusters: High-availability clusters spread a workload over a large number of servers and guarantee that the task is accomplished even if one of the nodes fails Computational clusters distribute a workload between a large number of servers in order to use the cumulative performance of each of its nodes NAS (Network Attached Storage) : NAS (Network Attached Storage) Introduction to NAS An "NAS" (Network Attached Storage) is a network storage device. An NAS is a separate storage server that can be easily attached to a company's network in order to serve the file server and provide fault-tolerant storage space. Presentation of an NAS An NAS is a separate server that has its own operating system. It generally has its own file system that hosts the operating system, as well as a series of independent disks that are used to host the data that is to be saved. SAN (Storage Area Network) : SAN (Storage Area Network) Introduction to SAN A "SAN" (Storage Area Network) is a complete storage network. A SAN is a complete architecture that groups together the following elements: A fibre channel broadband network or SCSI Dedicated interconnection equipment (switches, bridges, etc.) Network storage elements (hard drives) SAN: Advantages and Disadvantages : SAN: Advantages and Disadvantages The SAN's performance is directly linked to that of the type of network being used. In the case of a fibre channel network, the bandwidth is approximately 100 megabytes/second (1,000 megabits/second) and can be extended by increasing the number of access connections. A SAN's capacity can be extended almost limitlessly and can reach hundreds and even thousands of terabytes. Using a SAN, it is possible to share data between several network computers without sacrificing performance because SAN traffic is completely separate from user traffic. These are application servers that act as an interface between the data network (usually fibre channel) and the user network (usually Ethernet). On the other hand, it is much more costly to acquire a SAN than a NAS because it is a complete architecture that uses technology that is still very expensive. Often when a company figures out the TCO in terms of cost per byte, the cost can be more easily justified. Thanks : Thanks

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