Red Hat ENTERPRISE LINUX 4 - INTRODUCTION TO SYSTEM ADMINISTRATION Administration Manual page 88

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5.6.2.1. Basic Concepts
RAID is an acronym standing for Redundant Array of Independent Disks
RAID is a way for multiple disk drives to act as if they were a single disk drive.
RAID techniques were first developed by researchers at the University of California, Berkeley in the
mid-1980s. At the time, there was a large gap in price between the high-performance disk drives used
on the large computer installations of the day, and the smaller, slower disk drives used by the still-
young personal computer industry. RAID was viewed as a method of having several less expensive
disk drives fill in for one higher-priced unit.
More importantly, RAID arrays can be constructed in different ways, resulting in different character-
istics depending on the final configuration. Let us look at the different configurations (known as RAID
levels) in more detail.
5.6.2.1.1. RAID Levels
The Berkeley researchers originally defined five different RAID levels and numbered them "1"
through "5." In time, additional RAID levels were defined by other researchers and members of the
storage industry. Not all RAID levels were equally useful; some were of interest only for research
purposes, and others could not be economically implemented.
In the end, there were three RAID levels that ended up seeing widespread usage:
Level 0
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Level 1
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Level 5
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The following sections discuss each of these levels in more detail.
5.6.2.1.1.1. RAID 0
The disk configuration known as RAID level 0 is a bit misleading, as this is the only RAID level
that employs absolutely no redundancy. However, even though RAID 0 has no advantages from a
reliability standpoint, it does have other benefits.
A RAID 0 array consists of two or more disk drives. The available storage capacity on each drive is
divided into chunks, which represent some multiple of the drives' native block size. Data written to
the array is be written, chunk by chunk, to each drive in the array. The chunks can be thought of as
forming stripes across each drive in the array; hence the other term for RAID 0: striping.
For example, with a two-drive array and a 4KB chunk size, writing 12KB of data to the array would
result in the data being written in three 4KB chunks to the following drives:
The first 4KB would be written to the first drive, into the first chunk
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The second 4KB would be written to the second drive, into the first chunk
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The last 4KB would be written to the first drive, into the second chunk
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Compared to a single disk drive, the advantages to RAID 0 include:
Larger total size — RAID 0 arrays can be constructed that are larger than a single disk drive, making
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it easier to store larger data files
8. When early RAID research began, the acronym stood for Redundant Array of Inexpensive Disks, but over
time the "standalone" disks that RAID was intended to supplant became cheaper and cheaper, rendering the price
comparison meaningless.
Chapter 5. Managing Storage
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. As the name implies,