Instance Memory Structure

The size of these structures affects the performance of the Oracle database server and is controlled by initialization parameters. These initialization parameters can be categorized as memory parameters. When a database is created with DBCA, the memory parameters are automatically set to optimal values based on your specification of the database workload. However, as your database usage expands, you might find it necessary to alter the settings of the memory parameters. Oracle provides alerts and advisors to identify memory sizing problems and to help you determine appropriate values for memory parameters. Oracle’s memory structure consists of two memory areas known as:

• System Global Area (SGA): Allocated at instance startup, and is a fundamental component of   an Oracle Instance

• Program Global Area (PGA): Allocated when the server process is started

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Logical Storage Structure

Oracle Database allocates logical space for all data in the database. The logical units of database space allocation are data blocks, extents, segments, and tablespaces. At a physical level, the data is stored in data files on disk. The data in the data files is stored in operating system blocks.

The logical parts of the Oracle database are those structures within Oracle that determine where in the physical structures your table and index data reside. Since at the core of every operating system are mechanisms to access the physical data block defined on disk, Oracle must also have a logical structure that will relate down to that data block as your tables are created within the data files and as they grow over time. This is done by three logical structures that are kept for every object created in Oracle. These three structures are data blocks, extents, and segments.


Related Video:- Oracle DBA - Logical Database Structure

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Physical Storage Structure

The physical structure of an Oracle database is determined by the operating system files that provide the actual physical storage for database information.One characteristic of an RDBMS is the independence of logical data structures such as tables, views, and indexes from physical storage structures. Because physical and logical structures are separate, you can manage physical storage of data without affecting access to logical structures. For example, renaming a database file does not rename the tables stored in it. An Oracle database is a set of files that store Oracle data in persistent disk storage.The physical structure of an Oracle database includes only three types of files: control files, data files, and redo log files

Data files

A data file is a physical file on disk that was created by Oracle Database and contains data structures such as tables and indexes. A temp file is a data file that belongs to a temporary tablespace. The data is written to these files in an Oracle proprietary format that cannot be read by other programs.

Control files

A control file is a root file that tracks the physical components of the database.

Online redo log files

The online redo log is a set of files containing records of changes made to data.

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Database Storage Structure

An Oracle database is made up of physical and logical structures. Physical structures are those that can be seen and operated on from the operating system, such as the physical files that store data on a disk. Logical structures are created and recognized by Oracle Database and are not known to the operating system. The primary logical structure in a database, a tablespace, contains physical files. The applications developer or user may be aware of the logical structure, but is not usually aware of this physical structure. The DBA must understand the relationship between the physical and logical structures of a database.

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