RAID Technology
A major advance in secondary storage technology is represented by the development of RAID, which originally stood for Redundant Arrays of Inexpensive Disks. Lately, the "I" in RAID is said to stand for Independent. The RAID idea received a very positive endorsement by industry and has been developed into an elaborate set of alternative RAID architectures (RAID levels 0 through 6). We highlight the main features of the technology below.
The main goal of RAID is to even out the widely different rates of performance improvement of disks against those in memory and microprocessors. While RAM capacities have quadrupled every two to three years, disk access times are improving at less than 10 percent per year, and disk transfer rates are improving at roughly 20 percent per year. Disk capacities are indeed improving at more than 50 percent per year, but the speed and access time improvements are of a much smaller magnitude.
A second qualitative disparity exists between the ability of special microprocessors that cater to new applications involving processing of video, audio, image, and spatial data (see Chapter 23 and Chapter 27 for details of these applications), with corresponding lack of fast access to large, shared data sets.The natural solution is a large array of small independent disks acting as a single higher-performance logical disk. A concept called data striping is used, which utilizes parallelism to improve disk performance. Data striping distributes data transparently over multiple disks to make them appear as a single large, fast disk. Figure 05.03 shows a file distributed or striped over four disks. Striping improves overall I/O performance by allowing multiple I/Os to be serviced in parallel, thus providing high overall transfer rates. Data striping also accomplishes load balancing among disks. Moreover, by storing redundant information on disks using parity or some other error correction code, reliability can be improved. In Section 5.3.1 and Section 5.3.2, we discuss how RAID achieves the two important objectives of improved reliability and higher performance. Section 5.3.3 discusses RAID organizations.
A major advance in secondary storage technology is represented by the development of RAID, which originally stood for Redundant Arrays of Inexpensive Disks. Lately, the "I" in RAID is said to stand for Independent. The RAID idea received a very positive endorsement by industry and has been developed into an elaborate set of alternative RAID architectures (RAID levels 0 through 6). We highlight the main features of the technology below.
The main goal of RAID is to even out the widely different rates of performance improvement of disks against those in memory and microprocessors. While RAM capacities have quadrupled every two to three years, disk access times are improving at less than 10 percent per year, and disk transfer rates are improving at roughly 20 percent per year. Disk capacities are indeed improving at more than 50 percent per year, but the speed and access time improvements are of a much smaller magnitude.
A second qualitative disparity exists between the ability of special microprocessors that cater to new applications involving processing of video, audio, image, and spatial data (see Chapter 23 and Chapter 27 for details of these applications), with corresponding lack of fast access to large, shared data sets.The natural solution is a large array of small independent disks acting as a single higher-performance logical disk. A concept called data striping is used, which utilizes parallelism to improve disk performance. Data striping distributes data transparently over multiple disks to make them appear as a single large, fast disk. Figure 05.03 shows a file distributed or striped over four disks. Striping improves overall I/O performance by allowing multiple I/Os to be serviced in parallel, thus providing high overall transfer rates. Data striping also accomplishes load balancing among disks. Moreover, by storing redundant information on disks using parity or some other error correction code, reliability can be improved. In Section 5.3.1 and Section 5.3.2, we discuss how RAID achieves the two important objectives of improved reliability and higher performance. Section 5.3.3 discusses RAID organizations.
