MGMT 1050 Lecture Notes - Lecture 25: Standard Raid Levels
MGMT 1050 Lecture 25 Notes – Alternate Blocks
Introduction
• During reads, alternate blocks of the data are read from different drives
• Then combined to reassemble the original data.
• Thus, the access time for a multi-block read is reduced approximately by a factor equal
to the number of disk drives in the array.
• If a read failure occurs in one of the drives, the data can be read from another drive and
the bad block marked to prevent future use of that block, increasing system reliability.
• In critical applications, the data can be read from two, or even three, drives and
compared to increase reliability still further.
• When three drives are used, errors that are not detected by normal read failures can be
found using a method known as majority logic.
• This technique is particularly suitable for highly reliable computer systems known as
fault-tolerant computers.
• If the data from all three disks is identical, then it is safe to assume that the integrity of
the data is acceptable.
• If the data from one disk differs from the other two, then the majority data is used, and
the third disk is flagged as an error.
• The striped array uses a slightly different approach.
• In a striped array, a file segment to be stored is divided into blocks.
• Different blocks are then written simultaneously to different disks.
• This effectively multiplies the throughput rate by the number of data disks in the array.
• A striped array requires a minimum of three disk drives
• In the simplest configuration, one disk drive is reserved for error checking.
• As the write operation is taking place, the system creates a block of parity words from
each group of data blocks and stores that on the reserved disk.
• During read operations, the parity data is used to check the original data.
• There are five well-defined RAID standards, labeled RAID 1 through RAID 5
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