Oracle storage management is built on four basic entities:
1) data blocks, 2) extents, 3) segments, and 4) tablespaces.
These entities have a hierarchical relationship,
- extents contain data blocks,
- segments are made up of extents, and
- tablespaces are made up of segments.
Oracle Database manages the logical storage space in the data files of a database in a unit called a data block, also called an Oracle block or page. A data block is the minimum unit of database I/O.
At the physical level, database data is stored in disk files made up of operating system blocks. An operating system block is the minimum unit of data that the operating system can read or write.
In contrast, an Oracle block is a logical storage structure whose size and structure are not known to the operating system.
Figure 1 shows that operating system blocks may differ in size from data blocks. The database requests data in multiples of data blocks, not operating system blocks.
The tablespace, the highest level of physical organization in an Oracle database, is the interface between
- the physical storage of the database and
- the logical structures within the database.
The tables, indexes, and other objects within a database are assigned to a tablespace.
The tablespace is also the interface to the physical files managed by the underlying operating system. Each tablespace is associated with one or more physical files.
A tablespace is generally the smallest unit of physical storage that can be addressed by standard Oracle operations, such as backup and recovery.
This does not mean that you can ignore the underlying levels of space management. Because each of these levels of physical organization is dependent on the other,
the characteristics of each level of storage affect the higher levels.
Legacy Oracle Storage Concepts
The next lesson is about data blocks.