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238 Glossary
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Cache Input/Output (I/O)—A small amount of fast memory that holds
recently accessed data. Caching speeds subsequent access to the same data.
It is most often applied to processor-memory access, but can also be used to
store a copy of data accessible over a network. When data is read from or
written to main memory, a copy is also saved in cache memory with the
associated main memory address. The cache memory software monitors the
addresses of subsequent reads to see if the required data is already stored in
cache memory. If it is already in cache memory (a cache hit), it is read from
cache memory 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.
Channel—An electrical path for the transfer of data and control
information between a disk and a disk controller.
Consistency Check—An examination of the disk system to determine
whether all conditions are valid for the specified configuration (such as
parity).
Cold Swap—A cold swap requires that you turn the power off before
replacing defective hard drive in a disk subsystem.
Data Transfer Capacity—The amount of data per unit time moved through
a channel. For disk I/O, bandwidth is expressed in megabytes per second
(MB/s).
Degraded Drive—A disk drive that has become non-functional or has
decreased in performance.
Disk—A non-volatile, randomly addressable, rewritable mass storage device,
including both rotating magnetic and optical disks and solid-state disks, or
non-volatile electronic storage elements. It does not include specialized
devices such as write-once-read-many (WORM) optical disks, nor does it
include so-called RAM disks implemented using software to control a
dedicated portion of a host computer’s volatile random access memory.
Disk Array—A collection of disks from one or more disk subsystems
combined with array management software. It controls the disks and
presents them to the array operating environment as one or more virtual
disks.