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Channel
Any path used for the transfer of data and control of
information between storage devices and a storage
controller or I/O adapter. Also refers to one SCSI bus on a
disk array controller. Each disk array controller provides at
least one channel.
Cluster
A group of terminals or workstations attached to a common
control unit or server, or a group of several servers, that
share work and may be able to back each other up if one
server fails.
Cold Swap
The physical exchange of a replacement unit in a storage
system for a defective one. The exchange requires human
intervention and power must be removed from the storage
system in order to perform the exchange (compare to Hot
Swap and Auto Swap).
Conservative Cache
An operating mode in which system drives configured with
the write-back caching policy are treated as though they
were configured for write-through operation and the cache
is flushed.
Consistency Check
A process that verifies the integrity of redundant data. A
consistency check on a RAID 1 or RAID 0+1 configuration
(mirroring) checks if the data on drives and their mirrored
pair are exactly the same. For RAID Level 3 or RAID
Level 5, a consistency check calculates the parity from the
data written on the disk and compares it to the written
parity. A consistency check from utilities such as Global
®
Array Manager
™ (GAM) or RAID EzAssist™ give the
user the ability to have a discrepancy reported and
corrected. See also Parity Check.
Controller
An adapter card, RAID controller, or other module that
interprets and controls signals between a host and a
peripheral device.
D
Data Transfer Rate
© 2003, Adaptec, Inc.
The amount of data per unit of time moved through a
channel or I/O bus in the course of execution of an I/O
load, usually expressed in MBps.
Device Driver
A software program that controls a particular type of
device attached to a computer, such as a RAID subsystem,
printer, display, CD-ROM, disk drive, etc.
Disk
A non-volatile, randomly addressable, re-writable data
storage device, including rotating magnetic and optical
disks as well as solid-state disks or other electronic storage
elements.
Disk Array
A collection of disks from one or more commonly
accessible disk systems. Disk arrays, also known as RAID,
allow disk drives to be used together to improve fault
tolerance, performance, or both. Disk arrays are commonly
used on servers and are becoming more popular on
desktops and workstations. See also Array.
Disk Drive
A device for the electronic digital storage of information.
Disk Failure Detection
A RAID controller automatically detects SCSI disk
failures. A monitoring process running on the controller
checks, among other things, elapsed time on all commands
issued to disks. A time-out causes the disk to be "reset" and
the command to be retried. If the command times out
again, the controller could take the disk "offline." DAC960
controllers also monitor SCSI bus parity errors and other
potential problems. Any disk with too many errors will
also be taken "offline." See also Offline.
Disk Media Error Management
Controllers transparently manage disk media errors. Disks
are programmed to report errors, even ECC-recoverable
errors. If ECC RAM is installed, the controller will correct
ECC errors. When a disk reports a media error during a
read, the controller reads the data from the mirror (RAID 1
or 0+1), or computes the data from the other blocks (RAID
3, RAID 5), and writes the data back to the disk that
encountered the error. When a disk reports a media error
during a write, the controller issues a "reassign" command
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