gtpc3m21 | Concepts and Structures |
The generation of a database is an integral step in the TPF system generation process. A variety of information must be collected manually to initialize the online database. Some information is configuration dependent; for example, the number of types of terminals and workstations supported. Other information is application dependent; for example, the number flights in an airline's schedule. The process of creating the online database is summarized in the following sections.
File layout is a planning process and is a part of system generation.
The placement of programs and data records on a module is generally determined by the anticipated frequency of access. To maximize system performance, the most frequently accessed records are placed toward the center of the module, with the least frequently used records placed toward the outside. This tends to minimize the average seek time. The access frequency of most of the data records depends upon the design of the application. Using information from data collection and reduction, the user installation is able to modify the layout of the database to improve performance.
File layout also includes determining the size and location of various file pool types.
Fixed file allocation requires the use of the offline FACE table generation program (FCTBG), which is used to generate the FACE table (FCTB), based on the layout of the database as determined by file layout.
Fixed file records are initialized after using the offline system test compiler (STC) program to build pilot tapes that are used by a TPF program called the online data loader. The header of the records on a pilot tape contains the record type and ordinal number necessary to file (write) the record to the online database.
TPF disk modules must be initialized using a standard IBM MVS disk initialization program. This program checks for defective tracks, assigns alternate tracks when necessary, and initializes the volume labels according to TPF standards. Disk module initialization occurs as part of the system generation process.
Following disk module initialization, all online disk modules must be formatted with the TPF formatter program. Disk tracks are formatted for 4KB, 1055-byte, and 381-byte records. This formatting is based on input supplied by the user installation and must be consistent with the FACE table specifications. Disk module formatting is accomplished during the system generation process.
The final step in creating a fixed database is to load the pilot tapes to the online file modules. This function is provided by the online data loader for the TPF system. A system operator initiates a pilot tape load. Each record on the specified tape is read and filed at its appropriate location in the online database.