File Storage and Retrieval
File Storage and Retrieval
Information stored in a mass storage system is conceptually grouped into large units called files. A typical file may consist of a complete text document, a photograph, a program, a music recording, or a collection of data about the employees in a company. Mass storage devices dictate that these files be stored and retrieved in smaller, multiple byte units. For example, a file stored on a magnetic disk must be manipulated by sectors, each of which is a fixed predetermined size. A block of data conforming to the specific characteristics of a storage device is called a physical record. Thus, a large file stored in mass storage will typically consist of many physical records. In contrast to this division into physical records, a file often has natural divisions determined by the information represented. For example, a file containing information regarding a company’s employees would consist of multiple units, each consisting of the information about one employee. Or, a file containing a text document would consist of paragraphs or pages. These naturally occurring blocks of data are called logical records. Logical records often consist of smaller units called fields. For example, a logical record containing information about an employee would probably consist of fields such as name, address, employee identification number, etc. Sometimes each logical record within a file is uniquely identified by means of a particular field within the record (perhaps an employee’s identification number, a part number, or a catalogue item number). Such an identifying field is called a key field. The value held in a key field is called a key.
Logical record sizes rarely match the physical record size dictated by a mass storage device. In turn, one may find several logical records residing within a single physical record or perhaps a logical record split between two or more physical records. The result is that a certain amount of unscrambling is associated
Information stored in a mass storage system is conceptually grouped into large units called files. A typical file may consist of a complete text document, a photograph, a program, a music recording, or a collection of data about the employees in a company. Mass storage devices dictate that these files be stored and retrieved in smaller, multiple byte units. For example, a file stored on a magnetic disk must be manipulated by sectors, each of which is a fixed predetermined size. A block of data conforming to the specific characteristics of a storage device is called a physical record. Thus, a large file stored in mass storage will typically consist of many physical records. In contrast to this division into physical records, a file often has natural divisions determined by the information represented. For example, a file containing information regarding a company’s employees would consist of multiple units, each consisting of the information about one employee. Or, a file containing a text document would consist of paragraphs or pages. These naturally occurring blocks of data are called logical records. Logical records often consist of smaller units called fields. For example, a logical record containing information about an employee would probably consist of fields such as name, address, employee identification number, etc. Sometimes each logical record within a file is uniquely identified by means of a particular field within the record (perhaps an employee’s identification number, a part number, or a catalogue item number). Such an identifying field is called a key field. The value held in a key field is called a key.
Logical record sizes rarely match the physical record size dictated by a mass storage device. In turn, one may find several logical records residing within a single physical record or perhaps a logical record split between two or more physical records. The result is that a certain amount of unscrambling is associated