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MegaRAID Express 500 Hardware Guide
126
Glossary,
Continued
Parity
Parity is an extra bit added to a byte or word to reveal errors in storage (in RAM or disk)
or transmission. Parity is used to generate a set of redundancy data from two or more
parent data sets. The redundancy data can be used to reconstruct one of the parent data
sets. However, parity data does not fully duplicate the parent data sets. In RAID, this
method is applied to entire drives or stripes across all disk drives in an array. Parity
consists of dedicated parity, in which the parity of the data on two or more drives is stored
on an additional drive, and distributed parity, in which the parity data are distributed
among all the drives in the system. If a single drive fails, it can be rebuilt from the parity
of the respective data on the remaining drives.
Partition
An array virtual disk made up of logical disks rather than physical ones. Also known as
logical volume.
Physical Disk
A hard disk drive that stores data. A hard disk drive consists of one or more rigid magnetic
discs rotating about a central axle with associated read/write heads and electronics.
Physical Disk Roaming
The ability of some adapters to detect when hard drives have been moved to a
different slots in the computer, for example, after a hot swap.
Protocol
A set of formal rules describing how to transmit data, especially across a network. Low
level protocols define the electrical and physical standards to be observed, bit- and byte-
ordering, and the transmission and error detection and correction of the bit stream. High
level protocols deal with the data formatting, including the message syntax, the terminal-
to-computer dialogue, character sets, and sequencing of messages.
RAID
Redundant Array of Independent Disks (originally Redundant Array of Inexpensive
Disks) is an array of multiple small, independent hard disk drives that yields performance
exceeding that of a Single Large Expensive Disk (SLED). A RAID disk subsystem
improves I/O performance on a server using only a single drive. The RAID array appears
to the host server as a single storage unit. I/O is expedited because several disks can be
accessed simultaneously.
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