LSI MegaRAID Enterprise 1600 - Manual (Page 147)

Glossary 133
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.
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.
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