Overview of Logical Database Structures

Oracle® Database Concepts 10g Release 2 (10.2)：Overview of Logical Database Structures
Oracle Data Blocks

At the finest level of granularity, Oracle database data is stored in data blocks. One data block corresponds to a specific number of bytes
of physical database space on disk. The standard block size is specified by the 

DB_BLOCK_SIZE

initialization parameter. In addition, you can specify up to five other block sizes. A database uses and allocates free database space in Oracle data
blocks.

extent

The next level of logical database space is an extent. An extent is a specific number of contiguous data blocks, obtained in a single allocation,
used to store a specific type of information.

Segments

Above extents, the level of logical database storage is a segment. A segment is a set of extents allocated for a certain logical structure.
The following table describes the different types of segments.

Tablespaces

A database is divided into logical storage units called tablespaces, which group related logical structures together. For example, tablespaces
commonly group together all application objects to simplify some administrative operations.

Each database is logically divided into one or more tablespaces. One or more datafiles are explicitly created for each tablespace to physically store the data of all
logical structures in a tablespace. The combined size of the datafiles in a tablespace is the total storage capacity of the tablespace.

  Every Oracle Database contains a 

SYSTEM

 tablespace
and a 

SYSAUX

 tablespace. Oracle creates them automatically when the database is created. The system default is to create a smallfile tablespace, which is the traditional type of Oracle tablespace.
The 

SYSTEM

 and 

SYSAUX

 tablespaces are created as smallfile tablespaces.

Oacle also lets you create bigfile tablespaces. This allows Oracle Database to contain tablespaces made up of single large files rather than
numerous smaller ones. This lets Oracle Database utilize the ability of 64-bit systems to create and manage ultralarge files. The consequence of this is that Oracle Database can now scale up to 8 exabytes in size. With Oracle-managed files, bigfile tablespaces
make datafiles completely transparent for users. In other words, you can perform operations on tablespaces, rather than the underlying datafiles.

NOTE：Oracle数据库组成可以分为逻辑和物理体系架构：
     逻辑组成：tablespaces,segments, extents, data blocks，它们的关系

数据库包含至少一个表空间；
一个表空间包含一个或多个segment段；
一个segment段由多个区组成；
一个区由多个逻辑块组成；

物理组成： control files,online redo log files, and datafiles