When a datafile is assigned to a tablespace, Oracle formats the file into blocks for the actual storage of data. The size of each Oracle block is determined by the DB_BLOCK_SIZE parameter, which is specified when the database is created. The Oracle block is the basic unit of storage into which all Oracle objects (tables, indexes, rollback segments, etc.) are stored.
The Oracle block is divided into three distinct segments:
The fixed block header
The variable block header
The data storage area
Figure 11.2 illustrates the structure of an Oracle block.