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Redundant Array of Inexpensive Disks (RAID)
Even if only one full backup tape is needed for recovery of a system due to a hard disk failure, the time to recover a large amount of data can easily exceed the recovery time dictated by the organization. The goal of a Redundant Array of Inexpensive Disks (RAID) is to help mitigate the risk associated with hard disk failures. There are various RAID levels that consist of different approaches to disk array configurations. These differences in configuration have varying cost, with regard to both the number of disks required to achieve the configuration’s goals and capabilities in terms of reliability and performance advantages. Table 8.1 provides a brief description of the various RAID levels that are most commonly used.
Table 8.1
| RAID Level | Description |
|---|---|
| RAID 0 | Striped set |
| RAID 1 | Mirrored set |
| RAID 3 | Byte level striping with dedicated parity |
| RAID 4 | Block level striping with dedicated parity |
| RAID 5 | Block level striping with distributed parity |
| RAID 6 | Block level striping with double distributed parity |
Three critical RAID terms are: mirroring, striping, and parity.
- Mirroring is the most obvious and basic of the fundamental RAID concepts, and is simply used to achieve full data redundancy by writing the same data to multiple hard disks. Since mirrored data must be written to multiple disks, the write times are slower (though caching by the RAID controller may mitigate this). However, there can be performance gains when reading mirrored data by simultaneously pulling data from multiple hard disks. Other than read and write performance considerations, a major cost associated with mirroring is disk usage; at least half of the drives are used for redundancy when mirroring is used.
- Striping is a RAID concept that is focused on increasing the read and write performance by spreading data across multiple hard disks. With data being spread among multiple disk drives, reads and writes can be performed in parallel across multiple disks rather than serially on one disk. This parallelization provides a performance increase, but does not aid in data redundancy.
- Parity is a means to achieve data redundancy without incurring the same degree of cost as that of mirroring in terms of disk usage and write performance.
Exam Warning
While the ability to quickly recover from a disk failure is the goal of RAID, there are configurations that do not have reliability as a capability. For the exam, be sure to understand that not all RAID configurations provide additional reliability.