Database Concepts › Physical Structures › Segments › Files

Files

The database is stored as one or more files (data sets) on direct access devices. These files are formatted into a number of BDAM blocks or VSAM control intervals. Each direct access block corresponds to a database page so that data transfers are always accomplished one page at a time.

Sufficient file space must be allocated for all areas of the database. Areas may be mapped into the file(s) in any manner.

• The entire database may be mapped into one file.
• Each area may be mapped into a different file.
• Many areas may be mapped into one file.
• One area may be mapped into many files.

All areas mapping to a single file must be defined with the same page size, which is the file block size.

The Commonweather TESTHR database, for example, is mapped as one 4000-block file containing all three areas, as illustrated in the following figure. The PRODHR database is instead mapped to three files each containing one area. It would also be possible to map two of the areas to one file and the remaining area to its own file. Large areas are often mapped to several files. The only requirement is that each database page must correspond to one, and only one, direct access block.