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Disk arrays and levels of RAID

We have already dealt with the basic principles of organization of computer memory. Here more detail the problems of memory, because it is a major long-term data repository. As for RAM, for the most important characteristics of memory are speed data exchange and preservation. Obviously, because of the large size of memory loss of data stored in it because of equipment failure or refusal is a more serious problem than information loss of memory, and may require significantly more time for recovery processes calculations. Therefore, to the disk facing greater demands on reliability. One way to improve the reliability of memory is the oversupply of disk array to restore the original data, with failures and denials. One of the technologies improve the reliability of memory was called RAID - Redundant Array of Inexpensive Disks - redundant array of inexpensive disks. RAID technology is based on three main methods of recording and protecting information: * distribution segments of the sequence data on disks with a certain cyclical sequence, like discussed above for RAM; * mirror image discs; * computation of amounts. Consecutive placement allows you to create a large amount of memory volume and accelerate the execution of transactions recording and reading data, as the first segment involves recording data on the first disc, the second - for the second and so on. In this case array productivity increases, because the processor or controller input output starts to burn regular segment data for the next drive before the recording has finished the previous segment. Further gains in productivity achieved connecting different groups of disks to individual controllers. Mirror display storage and computation of control amounts lead to the emergence of redundant information that ensures the restoration of lost because of equipment failure or refusal of data. Various schemes implementation of disk arrays levels were called RAID. Level 0 RAID system is not a fault-and quite often such an organization of memory does not belong to the RAID array. In RAID 0 systems perform only alternating segments posting data on disks. They apply when the increased reliability of data storage is not very important, because failure of one drive led to the loss of the entire stored in an array of information. Such an organization of memory is used, for example, for tasks editing of images and all sorts of applications requiring high-capacity array and high speed operation of input-output. Fault-tolerant disk system is implemented on 1 RAID technology, which is a mirror image discs: data recorded on two or more disks simultaneously. In doing so, there is more than one copy of the data, and the level of redundancy for recording information on the two drives already is 100%. With the crash or when any one of these discs are read with his mirror. The disadvantage of RAID 1 is the high redundancy of equipment and possible problems with the replacement of a damaged disk, if implemented RAID 1 functionality programmatically, rather than through a special controller. In 2 RAID technology, commercial realization of which are virtually non-existent, provides for protection of data by correcting codes Hemminga. Records data distributed on several drives, and control levels ECC (Error-Correction Code) recorded on one or more designed specifically for this disc. The disadvantage of RAID 2 is a large share of ECC-disks in the array, which makes implementation of this architecture is quite expensive. In addition, the controllers of modern disk devices often have embedded error correction schemes. In RAID 3 data distributed on disks and information for aggregate data segments, located in the same sectors on different physical disks, is determined checksum code or parity, which are recorded on a separate disk. 3 RAID Arrays provide high speed data transmission in carrying out operations like reading and writing, and, compared with arrays RAID 2, they need fewer disks for storing parity. The disadvantage of this level RAID 3 is a sufficient complexity and the feasibility of hardware only way. Array RAID 4 increases the productivity of small amounts of data transmission through parallelism, giving the opportunity to perform more than one treatment entry to the group concluded in time unit. Logical blocks of information in a RAID 4 is not distributed among individual discs, and each individual unit lands on a separate disk. This makes it possible to perform several different requests for reading simultaneously. In RAID 5 arrays, as in RAID 4, disks alternately placed large blocks of data, but, unlike the previous system-level control information among the disk array. This small change has had a tremendous impact on the productivity of small arrays of information recording. If the transaction records could be planned so that to apply for relevant data and blocks them parity to different disks, the possibility of parallel implementation N / 2 records, where N - the number of disks in the group. The organization is equally high capacity, recording and reading as small or large volumes of information, making it the most attractive in cases of mixed uses. The following RAID levels designed to improve the reliability of data storage, but their implementation costs relatively expensive. RAID 6 is an expanded version of RAID 5, which provides for dual control parity stored information. If RAID 5 are invited to only one dimension of the matrix disk, the second dimension of which are sectors, the 6 RAID drives are combined in a two-dimensional array in such a way that the sector is the third dimension. Monitoring is carried out as the parity of lines, as in the systems level 5, and on columns, which in turn can to delaminate to enable parallel recording. If such an organization can overcome any waivers two disks and many refusals three disks. However, in carrying out the logical record six hits really happening to the disk - for the old data, for parity of lines and columns, as well as to record new data and new values of parity. For some applications with very high reliability requirements for such a redundancy may be acceptable, but RAID 6 architecture has a very low productivity records in connection with the need to calculate the amounts and additional controls for traditional supercomputing and processing transactions, the technique is not suitable. RAID Level 7 is a corporate decision of Storage Computer. It differs from other RAID levels that imply an asynchronous components of their work and independence of management. For this reason, controller RAID disk arrays 7 should run special real-time operating system. RAID 7 systems have a higher productivity compared to other levels of RAID arrays. They shared data on disks information, and reference information is stored on a separate disk. In order to improve productivity and record the reading centrally cached. In architectures RAID levels 10 and 0 +1 mirror image drives, as RAID 1 in the system, coupled with alternate accommodation segments of the data provided in an array of RAID level 0. These architecture developed when needed more productivity than that which provides an array of RAID 1. To implement system-level RAID 0 +1 or 10 requires a minimum of four disk. These systems cost quite expensive and poorly scaled. In RAID level 53 architecture combines RAID levels 0 and 3. For its implementation requires a minimum of 5 discs. RAID 53 alternately writes small segments of data for the first two drives, and information about parity on the third disc. The last two drives - the fourth and fifth contain the same data, alternately recorded large blocks without parity, as is done in the system RAID level 0. RAID technology is constantly evolving and improving. The most expensive system avtokonfigurirovaniya and acquire the ability to automatically select levels in real time. They use real-time operating systems, which analyze the flow of data and determine the most suitable environment for their accommodation. Budget same system usually focus on a narrow range of tasks and are based on a fixed standard levels or on the basis of policy decisions. There is also the introduction of functions RAID on server motherboards, or a combination of partial implementation at affordable motherboard controller and the motherboard. The most interesting modern technology implementations build fault-tolerant disk arrays are Dynamic Migration and AutoRAID. Dynamic Migration - dynamic migration, is a technology that provides automatic distribution of data in the system with multiple levels of RAID. Because most systems for a large part of the data has a low intensity use, the array is divided into two subsystems - RAID 5 and RAID 1. The data that are used most heavily, posted on an array architecture RAID 1, while all others at RAID 5. Thus, the system continually improves its performance by analyzing the activity data, and that is sufficiently high rate of use of disk space. AutoRAID technology developed by Hewlett Packard for storage systems and data processing. Systems based on AutoRAID constitute a fail-safe disk arrays, which are automatically selected for coding standard RAID data to the maximum extent appropriate current user queries. AutoRAID controller, unlike traditional systems, to display the addresses of blocks of data in the computer addresses of blocks of disk array uses algorithms dynamic rather than static. Dynamic algorithms allow data to locate any physical disk array. This makes it possible to broadcast RAID levels from one to another, with such an operation remains transparent to the host machine. Analyzing the type and intensity of requests, AutoRAID chooses the most suitable for serving algorithms, ensuring the highest possible speed possible while maintaining reliability storage subsystem. An important feature AutoRAID technology is the possibility of new and additional disks to balance the system. Under-balanced in this case refers to the distribution of data on all disks system to achieve maximum performance and utilization of the disks for active hot reserve.
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