Western Digital WDE2170 - Manual (Page 48)

Glossary WD Enterprise WDE2170/WDE4360
RELEASED 04-08-97 4079-001045 Rev. B 40
9. G LOSSARY
Active Termination - Works to control the impedance at the end of the SCSI bus by using a
voltage regulator. This reduces the susceptibility of the bus to noise, particularly when cables are
long or when many devices are connected to the bus. Because it is active, regulating the power
that it gets from the interface card, active termination is more stable than passive termination.
Adaptive Caching - The drive determines the environment in which it’s being used and
optimizes the way it handles commands and data.
Auto Park - Turning off the drive's power causes the WDE2170/WDE4360 drive to move the
read/write heads to a safe non-data landing zone and locks them in place.
Average Read Seek Time - Equal to the total time of a test divided by 50,000 random length
and random head seeks.
Block - Group of bytes handled, stored, and accessed as a logical data unit, such as an individual
file record.
Buffer - Temporary data storage area that compensates for a difference in data transfer rates
and/or data processing rates between sender and receiver.
Command Queuing - Feature that enables the drive to receive I/O processes from one or more
initiators.
Command Reordering - Feature that allows the drive to reorder I/O processes in the command
queue, thereby minimizing the seek time and rotational latency.
Correctable Error - Error that can be corrected by the use of Error Detection and Correction
algorithms.
Customer Configuration Code (CCC) - Located on the product label attached to the drive. This
code is revised only when changes affect the drive’s form, fit, or function.
Data Transfer Rate - Speed at which data transfers to and from the disk media (actual disk
platter); a function of the recording frequency. Typical units are bits per second (bps) or bytes per
second. Modern hard drives have an increasing range of disk transfer rates from the inner
diameter to the outer diameter of the disk. This is called a “zoned” recording technique.
Defect Management - General methodology of eliminating data errors on a recording surface by
mapping out known defects on the media.
Differential SCSI - Each signal consists of two lines called “–Signal” and “+Signal”.
Commands and data are carried over two sets of wires, and the difference is taken between each
set of signals. Two-wire signaling is a proven way to achieve reliable signal transmission in noisy
environments and over long distances.
ECC On-the-Fly - Hardware correction technique that corrects errors in the read buffer prior to
host transfer without any performance penalties. These error corrections are invisible to the host
system because they do not require assistance from the drive's firmware.
Embedded Servo Control - Design that generates accurate feedback information to the head
position servo system without requiring a full data surface (which is required with a “dedicated”
servo control design).
Error Correction Code - Mathematical algorithm that can detect and correct errors in a data
field by adding check bits to the original data.
Error Rate - Number of errors of a given type that occur when reading a specified number of
bits.