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Glossary 243
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 to eight physical
drives, RAID 1 requires exactly two physical drives, RAID 5 requires three to
eight physical drives and RAID 10 requires four, six or eight physical drives.
RAID 10 results when a RAID 1 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. 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,
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.
Redundancy—The provision of multiple interchangeable components to
perform a single function to cope with failures or errors. Redundancy
normally applies to hardware; a common form of hardware redundancy is
disk mirroring.
Replacement Disk—A disk available to replace a failed member disk in a
RAID array.