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Introduction
1-3
1.2 RAID Concepts
RAID Fundamentals
The basic idea of RAID (Redundant Array of Independent Disks) is to combine
multiple inexpensive disk drives into an array of disk drives to obtain performance,
capacity and reliability that exceeds that of a single large drive. The array of
drives appears to the host computer as a single logical drive.
Six types of array architectures, RAID 1 through RAID 6, were originally defined,
each provides disk fault-tolerance with different compromises in features and
performance. In addition to these five redundant array architectures, it has become
popular to refer to a non-redundant array of disk drives as a RAID 0 array.
Disk Striping
Fundamental to RAID technology is striping. This is a method of combining
multiple drives into one logical storage unit. Striping partitions the storage
space of each drive into stripes, which can be as small as one sector (512
bytes) or as large as several megabytes. These stripes are then interleaved
in a rotating sequence, so that the combined space is composed alternately
of stripes from each drive. The specific type of operating environment deter-
mines whether large or small stripes should be used.
Most operating systems today support concurrent disk I/O operations across
multiple drives. However, in order to maximize throughput for the disk subsystem,
the I/O load must be balanced across all the drives so that each drive can be
kept busy as much as possible. In a multiple drive system without striping, the
disk I/O load is never perfectly balanced. Some drives will contain data files that
are frequently accessed and some drives will rarely be accessed.