- American Megatrends, Inc. MegaRAID Express 500 Hardware Guide

Glossary
97
Glossary,
Continued
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 3 requires at least three physical drives, RAID 5 requires at least
three physical drives. RAID levels 10, 30, and 50 result when logical drives span
arrays. RAID 10 results when a RAID 1 logical drive spans arrays. RAID 30
results when a RAID 3 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 and in Windows NT its Power
Console. If a RAID 1 is being converted to a RAID 0, 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 is not as effective when accessing random
data.
Ready State A condition in which a workable hard 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, 3, 4, 5, or 6
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.
Cont’d