HP NetRAID 1 Controller - Manual (Page 82)

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distributed among all the disks in the system. If a single disk fails, it can be rebuilt from the parity of
the respective data on the remaining disks.
Partition: An array virtual disk made up of logical disks rather than physical ones. Also called
logical volume.
Physical Disk: A hard disk, one or more rigid magnetic discs rotating about a central axle with
associated read/write heads and electronics, used to store data.
Physical Disk Roaming: The ability of an adapter to keep track of a hot swap disk module that has
been moved to a different slot in the hot swap cages. Both slots must be controlled by the same
adapter.
Protocol: A set of formal rules describing how to transmit data, especially across a network. Low
level protocols define the electrical and physical standards to be observed, bit- and byte-ordering and
the transmission and error detection and correction of the bit stream. High level protocols deal with
the data formatting, including the syntax of messages, the terminal to computer dialogue, character
sets, sequencing of messages, etc.
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 using
only a single drive. The RAID array appears to the host NetServer 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 particular logical drive. It may increase the
performance of the logical drive, and it may decrease its usable capacity. Each logical drive must
have a RAID level assigned to it. 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, and RAID 50
results when a RAID 5 logical drive spans arrays.
Read-Ahead: A memory caching ability of some adapters to read sequentially ahead of requested
data and cache the further data in memory, anticipating that the further data will be requested. 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
therefore 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, 5, 10, 30, or 50 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 speed in MB/s at which the rebuild operation proceeds. Each adapter is assigned
a rebuild rate, which specifies the percentage of CPU resources to be devoted to rebuild operations.
A 5-drive (ST32550W 2 GB), RAID 5 array with a 64 KB stripe size takes 750 seconds to rebuild 8
GB of logical capacity, equivalent to 11 MB/s (logical capacity rebuild).
Reconstruct: The act of remaking a logical drive after changing RAID levels or stripe size.
Redundancy: The provision of multiple interchangeable components to perform a single function to
cope with failures and errors. Redundancy normally applies to hardware, e.g., using two or even
three computers to do the same job. The computers could all be active all the time, thus giving extra
performance through parallel processing and extra availability. Another type of redundancy could be
for one computer to be active while the others monitor its activity, ready to take over if it failed
(warm standby). Alternatively, the "spares" could be kept turned off and only switched on when
needed (cold standby). Another common form of hardware redundancy is disk mirroring.