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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.
RAID supports various configurations, including levels 0, 1, 4,
5, and linear. These RAID types are defined as follows:
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Level 0 — RAID level 0, often
called "striping," is a performance-oriented striped data mapping
technique. This means the data being written to the array is broken
down into strips and written across the member disks of the array,
allowing high I/O performance at low inherent cost but provides no
redundancy. The storage capacity of a level 0 array is equal to the
total capacity of the member disks in a Hardware RAID or the total
capacity of member partitions in a Software RAID.
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Level 1 — RAID level 1, or
"mirroring," has been used longer than any other form of RAID.
Level 1 provides redundancy by writing identical data to each
member disk of the array, leaving a "mirrored" copy on each disk.
Mirroring remains popular due to its simplicity and high level of
data availability. Level 1 operates with two or more disks that may
use parallel access for high data-transfer rates when reading but
more commonly operate independently to provide high I/O transaction
rates. Level 1 provides very good data reliability and improves
performance for read-intensive applications but at a relatively
high cost. The storage capacity of the level 1 array
is equal to the capacity of one of the mirrored hard disks in a
Hardware RAID or one of the mirrored partitions in a Software
RAID.
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Level 4 — Level 4 uses parity
concentrated on a single disk drive to
protect data. It is better suited to transaction I/O rather than
large file transfers. Because the dedicated parity disk represents
an inherent bottleneck, level 4 is seldom used without accompanying
technologies such as write-back caching. Although RAID level 4 is
an option in some RAID partitioning schemes, it is not an option
allowed in Red Hat Enterprise Linux RAID installations.
The storage capacity of Hardware RAID level 4 is equal to the
capacity of member disks, minus the capacity of one member disk.
The storage capacity of Software RAID level 4 is equal to the
capacity of the member partitions, minus the size of one of the
partitions if they are of equal size.
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Level 5 — This is the most common
type of RAID. By distributing parity across some or all of an
array's member disk drives, RAID level 5 eliminates the write
bottleneck inherent in level 4. The only performance bottleneck is
the parity calculation process. With modern CPUs and Software RAID,
that usually is not a very big problem. As with level 4, the result
is asymmetrical performance, with reads substantially outperforming
writes. Level 5 is often used with write-back caching to reduce the
asymmetry. The storage capacity of Hardware RAID level 5 is equal
to the capacity of member disks, minus the capacity of one member
disk. The storage capacity of Software RAID level 5 is equal to the
capacity of the member partitions, minus the size of one of the
partitions if they are of equal size.
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Linear RAID — Linear RAID is a
simple grouping of drives to create a larger virtual drive. In
linear RAID, the chunks are allocated sequentially from one member
drive, going to the next drive only when the first is completely
filled. This grouping provides no performance benefit, as it is
unlikely that any I/O operations will be split between member
drives. Linear RAID also offers no redundancy and, in fact,
decreases reliability — if any one member drive fails, the
entire array cannot be used. The capacity is the total of all
member disks.
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