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1.4 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.
Five types of array architectures, RAID 1 through RAID 5, 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 arrays.
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 determines 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 system, 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.
By striping the drives in the array with stripes large enough so that each record falls entirely within one
stripe, most records can be evenly distributed across all drives. This keeps all drives in the array busy
during heavy load situations. This situation allows all drives to work concurrently on different I/O
operations, and thus maximize the number of simultaneous I/O operations that can be performed by
the array.