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Memory Tuning: Make sure the SGA is tuned correctlyThe System Global Area(SGA) is a shared memory region that contains data and control information for one Oracle instance. An SGA and the Oracle background processes constitute an Oracle instance. Oracle allocates the system global area when an instance starts and deallocates it when the instance shuts down. Each instance has its own system global area.Users currently connect to an Oracle instance share the data in the system global area. For optimal performance,the entire system global area should be as large as possible( while still fitting in real memory avoid to causes paging the the swap file) to store as much data in memory as possible and minimize disk I/O. The information stored within the system global area is divided into several types of memory structures: These areas have fixed sizes and are created during instance up. Database Buffer Cache Database buffers of the system global area store the most recently used blocks of database data; the set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks.Because the most recently used data is kept in memory,less disk I/O is necessary and performance is improved. Redo Log Buffer The redo log buffer of system global area stores redo entries -- a log of changes made to the database.The redo entries stored in the redo log buffer are written to the online redo log files by the LGWR process,which is used if database recovery is necessary. Shared pool Comprised of the Library Cache (also known as the Shared SQL Area) and the Row Cache (also known as the Dictionary Cache). Library Cache: Contains the text, parsed form, and execution plan for SQL and PL/SQL statements. A single shared SQL area is used by multiple applications that issue the same statement,leaving more shared memory for other uses. Row Cache: Contains Oracle data dictionary tables, including the system catalog and privileges of all users. 2 3 |
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