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The inherent trade-off in RAID systems is between performance and reliability. You can employ two fundamental techniques, striping and mirroring the disk arrays, to improve disk performance and enhance reliability. Mirroring schemes involve complete duplication of the data, and while most of the nonmirrored RAID systems also involve redundancy, it is not as high as in the mirrored systems. The redundancy in nonmirrored RAID systems is due to the fact that they store the necessary parity information needed for reconstructing disks in case there is a malfunction in the array. The following sections describe the most commonly used RAID classifications. Except for RAID 0, all the levels offer redundancy in your disk storage system.

Strictly speaking, this isn t really a RAID level, since the striping doesn t provide you with any data protection whatsoever. The data is broken into chunks and placed across several disks that make up the disk array. The stripe here refers to the set of all the chunks. Let s say the chunk or stripe size is 8KB. If we have three disks in our RAID and 24KB of data to write to the RAID system, the first 8KB would be written to the first disk, the second 8KB would be written to the second disk, and the final 8KB would be written to the last disk. Because input and output are spread across multiple disks and disk controllers, the throughput of RAID 0 systems is quite high. For example, you could write an 800KB file over a RAID set of eight disks with a stripe size of 100KB in roughly an eighth of the time it would take to do the same operation on a single disk. However, because there is no built-in redundancy, the loss of a single drive could result in the loss of all the data, as data is stored sequentially on the chunks. RAID 0 is all about performance, with little attention paid to protection. Remember that RAID 0 provides you with zero redundancy.

In RAID 1, all the data is duplicated, or mirrored, on one or more disks. The performance of a RAID 1 system is slower than a RAID 0 system because input transactions are completed only when all the mirrored disks are successfully written to. The reliability of mirrored arrays is high, though, because the failure of one disk in the set doesn t lead to any data loss. The system continues operation under such circumstances, and you have time to regenerate the contents of the lost disks by copying data from the surviving disks. RAID 1 is geared toward protecting the data, with performance taking a back seat. Nevertheless, of all the redundant RAID arrays, RAID 1 still offers the best performance. It is important to note that RAID 1 means that you will pay for n number of disks, but you get to allocate only n/2 disks for your system, because all the disks are duplicated. Read performance improves under a RAID 1 system, because the data is scanned in parallel. However, there is slower write performance, amounting to anywhere from 10 to 20 percent, since both disks have to be written to each time.

RAID 2 uses striping with additional error detection and correction capabilities built in. The striping guarantees high performance, and error-correction methods are supposed to ensure reliability. However, the mechanism used to correct errors is bulky and takes up a lot of the disk space itself. This is a costly and inefficient storage system.

RAID 3 systems are also striped systems, with an additional parity disk that holds the necessary information for correcting errors for the stripe. Parity involves the use of algorithms to derive values that allow the lost data on a disk to be reconstructed on other disks. Input and output are slower on RAID 3 systems than on pure striped systems, such as RAID 0, because information also has to be written to the parity disk. RAID 3 systems can also only process one I/O request at a time. Nevertheless, RAID 3 is a more sophisticated system than RAID 2, and it involves less overhead than RAID 2. You ll only need one extra disk drive in addition to the drives that hold the data. If a single disk fails, the array continues to operate successfully, with the failed drive being reconstructed with the help of the stored error-correcting parity information on the extra parity drive. RAID 5 arrays with small stripes can provide better performance than RAID 3 disk arrays.