Dell PERC 4/DC - Manual (Page 91)

Multi-threaded
Having multiple concurrent or pseudo-concurrent execution sequences. Used to describe processes in systems. Multi-threaded processes allow throughput-
intensive applications to efficiently use a disk array to increase I/O performance.
Operating Environment
The operating environment includes the host system where the group of hard drives is attached, any I/O buses and controllers, the host operating system,
and any additional software required to operate the array. For host-based arrays, the operating environment includes I/O driver software for the member
disks.
Parity
Parity is an extra bit added to a byte or word to reveal errors in storage (in RAM or disk) or transmission. Parity is used to generate a set of redundancy data
from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets; however, parity data does not fully duplicate
the parent data sets. In RAID, this method is applied to entire drives or stripes across all hard drives in an array. Parity consists of dedicated parity, in which
the parity of the data on two or more drives is stored on an additional drive, and distributed parity, in which the parity data are distributed among all the
drives in the system. If a single drive fails, it can be rebuilt from the parity of the respective data on the remaining drives.
Partition
A separate logical area of memory or a storage device that acts as though it were a physically separate area.
Physical Disk
A hard drive that stores data. A hard drive consists of one or more rigid magnetic discs rotating about a central axle with associated read/write heads and
electronics.
Physical Disk Roaming
The ability of some adapters to detect when hard drives have been moved to a different slots in the system, for example, after a hot swap.
RAID
(Redundant Array of Independent Disks) An array of multiple independent hard disk drives that yields better performance than 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 at least one physical drive, RAID 1 requires two physical drives, RAID 5
requires at least three physical drives and RAID 10 requires at least four 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.
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, 5, 10, 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.
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.