Memory Structures that make up System Global Area

SGA Alternative Meaning
The System Global Area, or SGA, is the heart of any Oracle instance. All the various Oracle processes communicate with the SGA in one way or another.

The previous module introduced you to some of the major structures in the SGA. This module will go a bit deeper and discuss the topics in more detail. Here is a more detailed version of a MouseOver you saw earlier.

The database buffer cache, also called the buffer cache, is the memory area that stores copies of data blocks read from data files. A buffer is a main memory address in which the buffer manager temporarily caches a currently or recently used data block. All users concurrently connected to a database instance share access to the buffer cache.
Oracle Database uses the buffer cache to achieve the following goals:

1. Optimize physical I/O: The database updates data blocks in the cache and stores metadata about the changes in the redo log buffer. After a COMMIT, the database writes the redo buffers to the online redo log but does not immediately write data blocks to the data files. Instead, database writer (DBW) performs lazy writes in the background.
2. Keep frequently accessed blocks in the buffer cache and write infrequently accessed blocks to disk. When Database Smart Flash Cache (flash cache) is enabled, part of the buffer cache can reside in the flash cache. This buffer cache extension is stored on one or more flash disk devices, which are solid state storage devices that uses flash memory. The database can improve performance by caching buffers in flash memory instead of reading from magnetic disk.Use the DB_FLASH_CACHE_FILE and DB_FLASH_CACHE_SIZE initialization parameters to configure multiple flash devices. The buffer cache tracks each device and distributes buffers to the devices uniformly.

Before you move on, click the link below to read about SGA Structure Terms.

The data dictionary is a collection of database tables and views containing reference information about the database, its structures, and its users. Oracle Database accesses the data dictionary frequently during SQL statement parsing. The data dictionary is accessed so often by Oracle Database that the following special memory locations are designated to hold dictionary data:

1. Data dictionary cache: This cache holds information about database objects. The cache is also known as the row cache because it holds data as rows instead of buffers.
2. Library cache:All server processes share these caches for access to data dictionary information.

The redo log buffer is a circular buffer in the SGA that stores redo entries describing changes made to the database. A redo record is a data structure that contains the information necessary to reconstruct, or redo, changes made to the database by DML or DDL operations. Database recovery applies redo entries to data files to reconstruct lost changes. The database processes copy redo entries from the user memory space to the redo log buffer in the SGA. The redo entries take up continuous, sequential space in the buffer. The background process log writer process (LGWR) writes the redo log buffer to the active online redo log group on disk.