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.

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