SGA Structure Terms

Data Dictionary Cache	Holds frequently accessed data dictionary information
Redo Log Buffer	Holds redo log entries waiting to be written to disk
Library Cache	Contains the shared SQL area and PL/SQL code
Shared Pool	Contains memory structures related to SQL execution
Large Pool	An optional memory area used for backup and restore operations

Data Dictionary Cache

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:

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.
Library cache:All server processes share these caches for access to data dictionary information.

Redo Log Buffer

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.

Shared Pool consisting of I. Library Cache II.: 1) Data Dictionary Cache, 2) Server Result Cache, 3) Other, 4) Reserved Pool