Oracle Tablespaces, Segments, and Data Blocks Explained
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Oracle Tablespaces and Data Storage
System tablespaces and the required sysaux tablespace are created when the database is created and must be online.
The System tablespace is used for core functionality, for example, the data dictionary tables.
The auxiliary Sysaux tablespace is used for additional database components, such as the Enterprise Manager repository.
A tablespace accounts for one or more data files.
Data files belong to one tablespace.
User process: It begins when the user database server requests a connection to Oracle.
Process server: Connects to the Oracle instance and starts when a user establishes a session.
Background processes: Start the Oracle instance.
Oracle is a server management system database that provides an open, comprehensive, and integrated computer systems management. It consists of an Oracle instance and an Oracle database.
Segments exist in a tablespace.
Segments are formed by a collection of extensions.
Extensions are a block of data.
Data blocks are mapped to disk blocks.
Cache Buffers and Redo Log Buffer
Cache buffers database: Caches the data blocks that are retrieved from the database.
Redo log buffer: Caches redo information (used for instance recovery) until it can be written to the redo log files physically stored on disk.
Segments, Extensions, and Blocks
The database objects, such as tables and indices, are stored as segments in tablespaces. Each segment contains one or more extensions. An extension consists of contiguous data blocks, which means that each extension can only exist in a data file. The data blocks are the smallest unit of I/O in the database. When the database requests a set of data blocks from the operating system, it assigns it to a real file system or disk block in the storage device. For this reason, it is necessary to know the physical address of the data from the database. This also indicates that a data file can be segmented or duplicated across multiple discs.
The data block size can be defined when the database is created. The default size of 8 KB is appropriate for most databases. If the database supports a data warehouse application with large tables and indexes, a larger block size may be helpful.