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Oracle Arch

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  Oracle Server Architecture Oracle database consist of instance and database itself Oracle Instance: Oracle Instance is a combination of Memory structures and Background Processes used to manage the Database. There are two basic Memory structures in Oracle’s Instance.  1.   SGA (System Global Area) 2.   PGA (Program Global Area) 1.   SGA: SGA is a group of shared memory areas that are dedicated to an Oracle instance. Oracle allocates SGA is allocates when an instance starts up. De-allocates, when the instance shut down. The SGA consists of several memory structures a.   Buffer Cache. b.   Shared Pool. c.   Redolog Buffer. a.   Buffer Cache: The database buffer cache stores copies of data blocks that have been recently retrieved from the data files. ã It enables great performance gains when you obtain an update data. ã It is managed through a least recently used (LRU)algorithm.   When a query is being executed, the server process looks in the database buffer cache for any blocks it needed. If the blocks are not found in the buffer cache, then it reads the data from the dbf files and places a copy in the buffer cache. If any subsequent same requests are issued by the user, oracle gives the data from the buffer cache itself. b.   Shared Pool:   The shared pool is used to store the most recently executed SQL statements and the most recently used data definitions.    ã It consists of two me mory structures: The Library Cache The library cache is used to store shared SQL. Here the parse tree and the execution plan for every unique SQL statement are cached.Managed by Least Recently Used (LRU) algorithm. Size is determined by the Shared Pool sizing Data Dictionary Cache. Oracle stores information here about the logical and physical structure of the database. The data dictionary contains information such as the following: ã User information, such as user privileges   ã Integrity constraints defined for tables in the database   ã Names and data types of all columns in database tables   ã Information on space allocated and used for sche ma objects   c.   Redolog Buffer: Records all changes made to the database data blocks. Primary purpose is recovery. Changes recorded within are called redo entries. 1. Java Pool  : Used to store Java Code. 2. Large Pool  : Used to store large memory structures not Directly related to SQL statements processing, Such as data blocks copied during the backup and Restore operations. 2.PGA:   1.   PGA Stands for Program Global Area. 2.   PGA is allocated when a server process starts up. 3.   PGA contains data and control information for server process. 4.   PGA is used for user connects to an Oracle database and also session is created. 5.   Oracle creates one or more server process for users connected to database PGA Contains Sort area –  Used for any sorts required by SQL processing  Session information –  Includes user privileges. Cursor state –  Indicates stage of SQL processing. Stack space –  Contains session variables. Background Processes PMON        Pmon means Process Monitor.    Pmon automatic Process recovery.    Pmon mostly cleans up client-side failures.    Pmon performs process recovery when a user process fails.    Pmon cleans up (If user abnormally terminated) unwanted process.    Pmon also checks dispatcher process and server process and restarts them if they failed.    Pmon registers information about instance and dispatcher Process with network listener. Note : Performs Automatically resolves terminated Processes. Detects both user and server aborted database processes. Rolls back non committed (current)transactions of aborted process.

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Jul 23, 2017

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