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Glossary 239
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.