
Glossary, Continued
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. |
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.
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. |
| 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. |
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.
Replacement Unit A component or collection of components in a disk subsystem that are always replaced as a unit when any part of the collection fails. Typical replacement units in a disk subsystem includes disks, controller logic boards, power supplies, and cables. Also called a hot spare.
Cont’d
Glossary133