Page 65 - UNI 101 Computer Science Handout.

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Figure 7 Header for Windows Disk.

• Disk Arrays

In larger computer environments, with mainframe computers or large PCs that provide program and

data storage facilities for a network, it is common to group multiple disks together. Such a grouping of
two or more disk drives is called a disk array or a drive array. A disk array can be used to reduce overall

data access time by sharing the data among multiple disks and also to increase system reliability by

providing storage redundancy. The assumption made is that the number of blocks to be manipulated at

a given time is large enough and important enough to justify the additional effort and additional space
requirements. One useful type of disk array is known as RAID, which stands for Redundant Array of

Inexpensive Disks. (Some people say ‘‘Redundant Array of Independent Disks’’). There are two standard

methods of implementing a disk array. One is known as a mirrored array, and the other as a striped

array. A mirrored array consists of two or more disk drives.

In a mirrored array, each disk stores exactly the same data. During reads, alternate blocks of the data

are read from different drives, then combined to reassemble the original data. Thus, the access time for

a multiblock read is reduced approximately by a factor equal to the number of disk drives in the array.

If a read failure occurs in one of the drives, the data can be read from another drive and the bad block
marked to prevent future use of that block, increasing system reliability. In critical applications, the data

can be read from two, or even three, drives and compared to increase reliability still further. When three

drives are used, errors that are not detected by normal read failures can be found using a method known

as majority logic. This technique is particularly suitable for highly reliable computer systems known as
fault-tolerant computers. If the data from all three disks is identical, then it is safe to assume that the

integrity of the data is acceptable. If the data from one disk differs from the other two, then the majority

data is used, and the third disk is flagged as an error. The striped array uses a slightly different approach.

65 Academic Year 2025/2026