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The RAID Levels: An Overview
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to the original specification. This book will cover all of the RAID levels available to
Linux users, but will not cover obsolete implementations like RAID-2 and RAID-3.
Below you will find a concise overview of each RAID level. Chapter 2 covers each of
the RAID levels in more detail, including hybrid arrays that are built by combining
multiple RAID levels.
RAID-0: Striping
RAID-0 is also known as striping because data is interleaved across all drives in the
array. Each block of data is written in round-robin fashion to array disks until the
write operation is complete. Data is read in the same fashion. Since data transfer is
constantly shifted to a new disk, bottlenecks associated with reading and writing
data to a single disk are alleviated and performance dramatically improves. Striping
was not part of the original RAID specification and, technically speaking, is not a
RAID because it provides no mechanism for data redundancy. Nonetheless, the con-
cept of striping is also found in other RAID levels. For example, RAID-4 and RAID-5
(described below) use a combination of striping and recovery algorithms to achieve
improvements in performance while still offering redundancy.
RAID-1: Mirroring
RAID-1 (mirroring) stores an exact replica of all data on a separate disk or disks. This
practice provides complete data redundancy in the event of a disk failure. However,
because data must be written to disk more than once, there is a write performance
hit, which increases as you add disks. On the other hand, read operations can be
done in parallel so that read performance improves (compared to that of a single
disk), depending on the number of disks in the mirror.
RAID-4: Dedicated Parity
RAID-4 works similarly to striping. However, a dedicated drive is used to store par-
ity information. Every time data is written to an array disk, an algorithm generates
recovery information that is written to a specially flagged parity drive. In the event of
single disk failure, the algorithm can be reversed and missing data can be automati-
cally generated, based on the remaining data and the parity information.
RAID-5: Distributed Parity
RAID-5 is similar to RAID-4, except that the parity information is spread across all
drives in the array. This helps reduce the bottleneck inherent in writing parity infor-
mation to a single drive during each write operation.
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