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System Performance Tuning, 2nd Edition by Mike Loukides, Gian-Paolo D. Musumeci

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Chapter 6. Disk Arrays

I/O certainly has been lagging in the last decade.

Seymour Cray, 1976

Most of the improvements in disk technology have been made with the aim of increasing the capacity/price ratio. Although these changes have made mass storage much more affordable, they have brought about two fairly serious problems:

  • When a single disk can store tens of gigabytes of data, the reliability of an individual disk becomes a serious concern, as the failure of an individual disk results in the loss of large amounts of data.

  • Disk performance has lagged drastically behind capacity/price improvements.

To solve these problems, a great deal of effort has been put into designing methods of organizing sets of disks to enhance both reliability and performance. This has come to be known as RAID, which either stands for “Redundant Array of Inexpensive Disks” or “Redundant Array of Independent Disks,” depending on who you listen to. There are seven levels of RAID; each takes a different approach to solving these problems. The types of RAID are summarized in Table 6-1.

Table 6-1. A summary of RAID levels

RAID level

Organization

Strengths

Weaknesses

RAID 0

Striping

Very fast, simple

Low reliability

RAID 1

Mirroring

Fast, simple

Expensive

RAID 2

Reliability viaHamming codes

Reliable

Inflexible

RAID 3

Parity

Fast sequential access, reliable

Slow random access, implementation difficult

RAID 4

Parity

Average performance, reliable

Bottlenecks on dedicated parity disk

RAID ...

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