LSI Series 518 148

RAID	Redundant Array of Independent Disks. A data storage method in which
	data, along with parity information, is distributed among two or more hard
	disks (called an array) to improve performance and reliability. 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.
RAID Levels	A style of redundancy applied to a logical drive. It can increase the
	performance of the logical drive and can decrease usable capacity. Each
	logical drive must have a RAID level assigned to it. The RAID level drive
	requirements are: RAID 0 requires one or more physical drives, RAID 1
	requires exactly two physical drives, RAID 5 requires at least three
	physical drives. RAID levels 10 and 50 result when logical drives span
	arrays. RAID 10 results when a RAID 1 logical drive spans arrays. RAID
	50 results when a RAID 5 logical drive spans arrays.
RAID Migration	RAID migration is used to move between optimal RAID levels or to
	change from a degraded redundant logical drive to an optimal RAID 0.
	In Novell, the utility used for RAID migration is MEGAMGR; in
	Windows NT, it is Power Console Plus. If a RAID 1 array is being
	converted to a RAID 0 array, instead of performing RAID migration, one
	drive can be removed and the other reconﬁgured on the controller as a
	RAID 0. This is due to the same data being written to each drive.
Read-Ahead	A memory caching capability in some adapters that allows them to read
	sequentially ahead of requested data and store the additional data in
	cache memory, anticipating that the additional data will be needed soon.
	Read-Ahead supplies sequential data faster, but it is not as effective
	when accessing random data.
Ready State	A condition in which a workable disk drive is neither online nor a hot
	spare and is available to add to an array or to designate as a hot spare.
Rebuild	The regeneration of all data from a failed disk in a RAID level 1, or 5
	array to a replacement disk. A disk rebuild normally occurs without
	interruption of application access to data stored on the array virtual disk.
Rebuild Rate	The percentage of CPU resources devoted to rebuilding.
Reconstruct	The act of remaking a logical drive after changing RAID levels or adding
	a physical drive to an existing array.

C-6Glossary