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A system global area (SGA)  is a group of shared memory structures that contain data and control information for one Oracle database instance. If multiple users are concurrently connected to the same instance, then the data in the instance's SGA is shared among the users. Consequently, the SGA is sometimes called the shared global area.Oracle SGA is further divided into various memory structures where each of these have specific function to perform

An Oracle instance comprises of  System Global Area [ SGA ] and Oracle processes .All Memory Allocation in the SGA is done automatically on start of Oracle Instances.and the operating system reclaims the memory when you shut down the instance. Each instance has its own SGA.The Memory Allocation is done by automatics memory management (AMM) feature. This feature has some overhead,

The SGA is read/write. All users connected to a multiple-process database instance can read information contained within the instance's SGA, and several processes write to the SGA during execution of Oracle.

The System Global Area [SGA] Mainly Comprises Of:

• Database buffer cache : This is a chache memory which stores information which can be provided to oracle (Query) and helps in boosting the perfomance by avoiding a physical read and stores information blocks. The data stored inside the buffer cache is dependent excution of query and how frequently such data is being querries.Oracle Uses various Algorithms like MRU (Most recently used & LRU (Least recently used) to store and free data blocks in the buffer.

• Redo log buffer : The redo log buffer is a circular buffer in the SGA that holds information about changes made to the database. This information is stored in redo entries. Redo entries contain the information necessary to reconstruct, or redo, changes made to the database by INSERT, UPDATE, DELETE, CREATE, ALTER, or DROP operations. Redo entries are used for database recovery, if necessary.

  Redo entries are copied by Oracle database processes from the user's memory space to the redo log buffer in the SGA. The redo entries take up continuous, sequential space in the buffer. The background process LGWR writes the redo log buffer to the active redo log file (or group of files) on disk.

• Shared pool :

  The shared pool portion of the SGA contains the library cache, the dictionary cache, buffers for parallel execution messages, and control structures.

  The total size of the shared pool is determined by the initialization parameter SHARED_POOL_SIZE. The default value of this parameter is 8MB on 32-bit platforms and 64MB on 64-bit platforms. Increasing the value of this parameter increases the amount of memory reserved for the shared pool.

• Java pool : Java pool memory is used in server memory for all session-specific Java code and data within the JVM. Java pool memory is used in different ways, depending on what mode the Oracle server is running in.

  The Java Pool Advisor statistics provide information about library cache memory used for Java and predict how changes in the size of the Java pool can affect the parse rate.
  

• Large pool (optional)

• stream pool : In a single database, you can specify that Streams memory be allocated from a pool in the SGA called the Streams pool. To configure the Streams pool, specify the size of the pool in bytes using the STREAMS_POOL_SIZE initialization parameter. If a Streams pool is not defined, then one is created automatically when Streams is first used.

• Data dictionary cache :

• Other miscellaneous information  

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