The logical units of database space allocation are data blocks, segments, tablespaces and extents, segments. Initially, the data is stored in data files on disk at physical level and then the data in the data files is stored in operating system blocks at logical level.
The Oracle RDBMS stores data logically in the form of table spaces.Tablespaces can consists various types of memory segments. For an installation of Oracle 11g, it allow us to create bigfile tablespace. The size can achieve up to 128TB and completely transparent to DBA. Therefore, DBA can manage the tablespace without worry about the size and structure of underlying datafiles. Segments is a group that comprise one or more extents. There are four types of segments in Oracle database such as data segments, index segments, temporary segments and rollback segments. As a result, this typically the smallest unit of storage that an end user of database will deal with. Extents is next level in database after segments. It comprise groups of contiguous data blocks when we enlarge a database object. Data blocks form the basic units of data storage and it is the smallest unit storage in Oracle database. A block is usually a multiple of the operating system block size to facilitate efficient disk I/O. The default block size is specified by the Oracle initialization parameter DB_BLOCK_SIZE. As many as four other block sizes may be defined for other tablespaces in the database, although the blocks in the SYSTEM, SYSAUX, and any temporary tablespaces must be of the size