LIS 505 - Storage

mass storage devices

Devices for storing large amounts of data in a relatively permanent form. Also called secondary storage devices or auxiliary storage devices.

read-write heads

The parts of a disk or tape drive that read and write information on the medium. ComputerUser

disk drives

Devices that read data from, and write data to, a disk.

magnetic disks

Disks on which data are encoded as tiny magnetic regions.

floppy disks

Soft, removable magnetic disks. Also called diskettes or floppies.

hard disks

Hard magnetic disks, usually not easily removed and faster and more capacious than floppy disks. A single disk drive may contain several platters, which look like a single disk to the software.
head crashes
Serious disk drive malfunctions, usually caused by a read-write head's scratching or burning a disk.
RAID
Short for Redundant Array of Independent (or Inexpensive) Disks. A method for using two or more disk drives in combination to improve error recovery and speed. Used on some servers.
disk mirroring
A technique in which duplicate data are written to two disks simultaneously.
disk striping
A technique in which data are broken up into units which are spread across available disks.

tracks

Rings on disks or platters on which data can be written.

sectors

The smallest units that can be accessed on disks. Portions of tracks. Modern hard disks divide tracks into different numbers of sectors depending on the radius of the track, in a scheme called zoned-bit recording or zone recording.

cylinders

Sets of tracks on a single disk drive all with the same radius. All the tracks that can be read in a single position of the read-write heads.

clusters

Groups of one or more sectors assigned by an operating system. . The smallest units that can be assigned to a file.

access time

Time taken to locate a piece of data and make it available to a computer. . For a disk drive, the portion of this time required to position the read-write heads is called seek time, and latency (or rotational latency or rotational delay) is the additional time taken for the right sector in the track to come under the read-write heads.
data transfer rate
Speed at which data can be transferred. . Typically measured in megabytes per second for hard drives.

optical disks

Disks read from (and sometimes written to) with lasers. .

read-only optical disks

CD-ROMs
Short for compact disk - read-only memory. A standard type of read-only optical disk usually with a capacity of 650 megabytes. .
DVD-ROMs
A newer standard type of read-only optical disk with a capacity of 4.7 gigabytes or higher. .

WORM disks

Short for write-once read-many. .
CD-R
Short for compact disk - recordable. A standard write-once read-many technology compatible with CD-ROMs. .

CD-RW

Short for compact disk - rewritable. A standard write-many read-many technology, compatible with CD-R and partially with CD-ROMs. .

tape drives

Devices for writing to and reading from tapes. .

erase head

A device that erases data on tapes before new data are written. ComputerUser

tapes

Long magnetically coated strips of plastic on which data can be recorded. . More exactly, magnetic tape or magtape.

characters

Printable symbols with phonetic, pictographic, or ideographic meaning, usually text elements (e.g., letters), numerals, or punctuation marks. A single character may have a variety of different printed forms, known as glyphs. whatis.com. Characters are often equated with bytes; but in Unicode each character is represented by two bytes.

fields

Divisions of records in database and similar files. . Typically, each field corresponds to an attribute of an entity or of a relationship between entities.

records

Divisions of database tables or files. . Typically, each record corresponds to an instance of either a type of entity or a type of relationship between entities.

files

Named collections of stored data. A file may contain a program or data to be used with one or more programs. .

databases

Collections of data organized so that their contents (typically large and at least moderately complex) can be accessed, managed, and updated easily. whatis.com.

keys

In a database management system, fields (or combinations of fields) that can be used to sort records. The primary key must have a unique value for each record; a foreign key must correspond to a primary key in another set of records in the database. .

file organization

(For a more detailed comparison of file structures, see LIS 558 - File structures.)

sequential file organization

Organization of files for sequential access, in which records are read or written in sequence, from beginning to end. .

random file organization

Organization that allows random access, reading or writing at any point in a file by jumping directly to that point without reading or writing intervening parts of the file. Also called direct file organization. .

hashing

Producing relatively small numeric values from longer texts, with the aim being to minimize cases where different texts produce the same numeric value. Useful for determining where to place and find records in a random access file. .

indexed file organization

File organization including a list of keys, each with a pointer to a corresponding record. .

batch processing

Processing in which the computer executes a series of noninteractive tasks all at one time. . A typical batch process would involve taking a master file and applying a a transaction file containing a list of changes to create a new master file.

transaction processing

Processing in which the computer responds immediately to user requests. .
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Last updated October 29, 2002.
This page maintained by Prof. Tim Craven
E-mail (text/plain only): craven@uwo.ca
Faculty of Information and Media Studies
University of Western Ontario,
London, Ontario
Canada, N6A 5B7