A file is organized logically as a sequence of records. Records are mapped onto disk blocks. Files are provided as a basic construct in operating systems, so we assume the existence of an underlying file system .
Blocks are of a fixed size determined by the operating system.
Record sizes vary. In relational database, tuples of distinct relations may be of different sizes. One approach to mapping database to files is to store records of one length in a given file. An alternative is to structure files to accommodate variable-length records.