Availability is how well a system can work in times of a failure If a system is

Availability is how well a system can work in times

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Availability is how well a system can work in times of a failure. If a system is able to work even in the presence of a failure of one or more system components, the system is said to be available. Redundancy improves the availability of a system, but cannot improve the reliability. Reliability can only be increased by improving manufacturing technologies, or by using fewer individual components in a system. Redundancy has costs Every time there is a write operation, there is a change of data. This change also has to be reflected in the disks storing redundant information. This worsens the performance of writes in redundant disk arrays significantly, compared to the performance of writes in non-redundant disk arrays. Moreover, keeping the redundant information consistent in the presence of con- current I/O operation and the possibility of system crashes can be difficult. The Need for RAID The need for RAID can be summarised as: An array of multiple disks accessed in parallel will give greater throughput than a single disk, and Redundant data on multiple disks provides fault tolerance. Provided that the RAID hardware and software perform true parallel accesses on multiple drives, there will be a performance improvement over a single disk. 14
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With a single hard disk, you cannot protect yourself against the costs of a disk failure, the time required to obtain and install a replacement disk, reinstall the operating system, restore files from backup tapes, and repeat all the data entry performed since the last backup was made. With multiple disks and a suitable redundancy scheme, your system can stay up and running when a disk fails, and even while the replacement disk is being installed and its data restored. We aim to meet the following goals: maximise the number of disks being accessed in parallel, minimise the amount of disk space being used for redundant data, and minimise the overhead required to achieve the above goals. Data Striping Data striping, for improved performance, transparently distributes data over multiple disks to make them appear as a single fast, large disk. Striping im- proves aggregate I/O performance by allowing multiple I/O requests to be serviced in parallel. Multiple independent requests can be serviced in parallel by separate disks. This decreases the queuing time seen by I/O requests. Single multi-block requests can be serviced by multiple disks acting in co- ordination. This increases the effective transfer rate seen by a single request. The performance benefits increase with the number of disks in the array, but the reliability of the whole array is lowered. Most RAID designs depend on two features: the granularity of data interleaving, and 15
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the way in which the redundant data is computed and stored across the disk array.
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