Information Storage and Management: Storing, Managing, and Protecting Digital Information in Classic, Virtualized, and Cloud Environments, Second Edition

Chapter 3

Data Protection: RAID

Key Concepts

Hardware and Software RAID

Striping, Mirroring, and Parity

RAID Levels

RAID Write Penalty

Hot Spares

In the late 1980s, rapid adoption of computers for business processes stimulated the growth of new applications and databases, significantly increasing the demand for storage capacity and performance. At that time, data was stored on a single large, expensive disk drive called Single Large Expensive Drive (SLED). Use of single disks could not meet the required performance levels because they were capable of serving only a limited number of I/Os.

Today's data centers house hundreds of disk drives in their storage infrastructure. Disk drives are inherently susceptible to failures due to mechanical wear and tear and other environmental factors, which could result in data loss. The greater the number of disk drives in a storage array, the greater the probability of a disk failure in the array. For example, consider a storage array of 100 disk drives, each with an average life expectancy of 750,000 hours. The average life expectancy of this collection in the array, therefore, is 750,000/100 or 7,500 hours. This means that a disk drive in this array is likely to fail at least once in 7,500 hours.

RAID is an enabling technology that leverages multiple drives as part of a set that provides data protection against drive failures. In general, RAID implementations also improve the storage system performance by serving I/Os from multiple disks ...