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Contemporary disc systems of RAID

The emergence and development of the concept of RAID related to the increase in the use of data, the need to ensure the most rapid access and high reliability storage. The amount of stored information has been calculated by many hundreds of terabytes. It is now essential tool storage, providing quick access to information is on magnetic disk drives, characteristics which in recent years has grown. However, in comparison with purely electronic devices (CPU, RAM, controllers, etc.) because of productivity growth electromechanical nature drives markedly lower. Since the beginning of 80 years, the central processor speed increased at an average of five, and by some estimates, even tenfold every three years. External drive capacity doubled, falling in price by half to about every three years. However, the access time for the period improved according to different estimates by only 10-50%. As a result, external drives have become "bottleneck ". Alas, soon no innovative solutions to realize the external drive without the shortcomings of magnetic disks. For example, optical storage, capable of providing much more specific capacity, suffer the same drawback: the availability elektro-mehanicheskih knots. The flow of user queries very unevenly distributed across the disk memory. The formation of the so-called "hot spots" - areas to which the vast majority of treatment takes place, while the rest of the memory is very low load. This situation applies known "rule 80/20" in most cases, 80% of hits implemented to a limited area that constitutes only 20% of capacity memory. If this "hot spot" located on the same disk, external memory subsystem performance will be very low, because the consecutive requests for most of the time will be expected to end operations addition of the ball and finding the desired sector on the disk. One of the approaches proposed for reducing the adverse effects of rule 80/20, "is to use a more expensive device with a significantly better performance for storage" hot files ". However, few in a position to determine the likely nabolee set such files, the more so that, depending on the type of tasks, the time of day, day of week and other factors will inevitably change the picture. The most common solution to this problem is a logical allocation of disk space to recruit (array) of relatively small and relatively inexpensive disk devices with an average search time. "A hot spot" located in the small fragments of the drive, made more uniform than their load. With the apparent attractiveness of such a solution, and it has a very serious flaw: the increasing number of disks in the array, and proportionally increases the likelihood of rejection. In other words, in parallel with the increasing number of working time developments in the drive waiver system as a whole is declining, its reliability has become unacceptably low. Productivity growth in the number of the drive will increase reliability and decrease. Increase the reliability possible by the introduction of redundant capacity, providing an opportunity to restore the destroyed data. RAID levels.
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