RAID Technology Overview - September 2007

Glossary
array A set of physical disks configured into one or more logical drives. Arrayed disks have significant
performance and data protection advantages over non-arrayed disks.
array capacity
expansion
See capacity expansion.
Auto-Reliability Monitoring (ARM)
Also known as surface analysis. A fault management feature that scans physical disks for bad
sectors. Data in the faulty sectors remaps onto good sectors. Also checks parity data consistency
for disks in RAID 5 or RAID ADG configurations. Operates as a background process.
Automatic Data
Recovery
A process that automatically reconstructs data from a failed disk and writes it onto a replacement
disk. Automatic Data Recovery time depends on several factors, but HP recommends that you
allow at least 15 minutes per gigabyte. Also known as rebuild.
cache A high-speed memory component, used to store data temporarily for rapid access.
capacity
expansion
The addition of physical disks to an existing disk array, and redistribution of existing logical
drives and data over the enlarged array. The size of the logical drives does not change. Also
known as array capacity expansion.
capacity
extension
The enlargement of a logical drive without disruption of data. There must be free space on the
array before capacity extension can occur. If necessary, create free space by deleting a logical
drive or by carrying out a capacity expansion. Also known as logical drive capacity extension.
data guarding See RAID.
data striping Writing data to logical drives in interleaved chunks (by byte or by sector). Data striping improves
system performance by distributing data evenly across all physical disks in the array, but has
no fault tolerance
drive mirroring Duplicating data from one disk onto a second disk. Mirroring provides fault tolerance, but can
only recover from failure of one physical disk per mirrored pair.
Error Correction
and Checking
(ECC) memory
A type of memory that checks and corrects single-bit or multi-bit memory errors (depending
on configuration) without causing the server to halt or to corrupt data.
fault tolerance The ability of a server to recover from physical disk hardware problems without interrupting
server performance or corrupting data. Hardware RAID is most commonly used, but there are
other types of fault tolerance, including controller duplexing and software-based RAID.
hot spare See online spare.
interim data
recovery
If a disk fails in RAID 1, 1+0, 5 or ADG, the system still processes I/O requests, but at a reduced
performance level.
logical drive A group of physical disks, or part of a group, that behaves as one storage unit. Each constituent
physical disk contributes the same storage volume to the total volume of the logical drive. A
logical drive has performance advantages over individual physical disks. Also known as a
logical volume.
logical drive
capacity
extension
See capacity extension.
online spare A fault-tolerant system that normally contains no data. When any other disk in the array fails,
the controller automatically rebuilds the data that was on the failed disk onto the online spare.
Also known as a hot spare.
physical disk A random-access storage device. In traditional non-arrayed storage, one physical disk typically
contains a single logical drive. In RAID configurations, multiple disks are combined to form a
single logical drive.
rebuild See Automatic Data Recovery.
23