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Glossary
immediately and the main memory read is aborted (or not started). If the data is not
cached (a cache miss), it is fetched from main memory and saved in cache memory.
C
ACHING
— The process of utilizing a high speed memory buffer, referred to as a
“cache,” in order to speed up the overall read or write performance. This cache can be
accessed at a higher speed than a disk subsystem. To improve read performance, the
cache usually contains the most recently accessed data, as well as data from adjacent
disk sectors. To improve write performance, the cache may temporarily store data in
accordance with its Write-Cache Enable policies. See the definition of Write-Cache
Enable for more information.
C
HANNEL
— A link that transports data point-to-point.
C
ONSISTENCY
C
HECK
— An operation to verify that all stripes in a virtual disk with a
redundant RAID level are consistent and to automatically fix any errors. For RAID 5
arrays, a consistency check verifies correct parity data for each stripe. For RAID 1 and
RAID 10 arrays, this operation verifies correct mirror data for each stripe.
C
ONTROLLER
— A chip that controls the transfer of data between the microprocessor
and memory or between the microprocessor and a peripheral device such as a physical
disk or the keyboard. In Storage Management, the hardware or logic that interacts with
storage devices to write and retrieve data and perform storage management. RAID
controllers perform RAID functions such as striping and mirroring to provide data
protection.
D
ISK
— A non-volatile, randomly addressable, rewriteable mass storage device,
including both rotating magnetic and optical storage devices and solid-state storage
devices, or non-volatile electronic storage elements.
D
ISK
A
RRAY
— A grouping of physical disks that are connected to the RAID
controller. The RAID controller can group physical disks on one or more channels into
an array.
D
ISK
M
IGRATION
— Disk migration refers to moving a virtual disk or a hot spare
from one controller to another by detaching the physical disks and re-attaching them
to the new controller.
D
ISK
R
OAMING
— Moving disks from one slot to another on a controller.
D
ISTRIBUTED
P
ARITY
— Parity involves 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 rebuild one of the parent data sets. In distributed parity, the parity data are
distributed among all the physical disks in the system. If a single physical disk fails, it
can be rebuilt from the parity of the applicable data on the remaining physical disks.
F
AILED
P
HYSICAL
D
ISK
— A physical disk that has ceased to function, that
consistently functions improperly, or that is inaccessible.
A5_bk0.book Page 104 Thursday, February 10, 2011 8:34 PM