Disk Duplexing—A variation on disk mirroring in which a second disk adapter or host adapter and redundant hard drives are present.

Disk Mirroring—Writing duplicate data to more than one (usually two) hard disks to protect against data loss in the event of device failure. Disk mirroring is a common feature of RAID systems.

Disk Spanning—Disk spanning allows multiple hard drives to function like one big drive. Spanning overcomes lack of disk space and simplifies storage management by combining existing resources or adding relatively inexpensive resources. For example, four 400 MB hard drives can be combined to appear to the operating system as one single 1600 MB drive. See also Array Spanning and Spanning.

Disk Striping—A type of disk array mapping. Consecutive stripes of data are mapped round-robin to consecutive array members. A striped array (RAID level 0) provides high I/O performance at low cost, but provides lowers data reliability than any of its member disks.

Disk Subsystem—A collection of disks and the hardware that connects them to one or more host computers. The hardware can include an intelligent controller, or the disks can attach directly to a host computer.

Double Buffering—A technique that achieves maximum data transfer bandwidth by constantly keeping two I/O requests for adjacent data outstanding. A software component begins a double-buffered I/O stream by issuing two requests in rapid sequence. Thereafter, each time an I/O request completes, another is immediately issued. If the disk subsystem is capable of processing requests fast enough, double buffering allows data to be transferred at the full-volume transfer rate.

Failed Drive—A drive that has ceased to function or consistently functions improperly.

Firmware—Software stored in read-only memory (ROM) or Programmable ROM (PROM). Firmware is often responsible for the behavior of a system when it is first turned on. A typical example would be a monitor program in a computer that loads the full operating system from disk or from a network and then passes control to the operating system.

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