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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.

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