Managing an oracle instance

Using Online Redo Log Files Online redo log files provide the means to redo transactions in the event of a database failure. Every transaction is written synchronously to the Redo Log Buffer, then gets flushed to the online redo log files in order to provide a recovery mechanism in case of media failure. (With exceptions such as direct load inserts in objects with the NOLOGGING clause enabled.) This includes transactions that have not yet been committed, undo segment information, and schema and object management statements.

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Initialization Parameter Files In order to start an instance and open the database, you must connect as SYSDBA and enter the STARTUP command. The Oracle server will then read the initialization parameter file and prepare the instance according to the initialization parameters contained within. Note: You must have SYSDBA privilege. Authentication and the SYSDBA privilege will be discussed in later lessons.

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Overview of Primary Components The Oracle architecture includes a number of primary components, which are discussed further in this lesson. Oracle server: There are several files, processes, and memory structures in an Oracle server; however, not all of them are used when processing a SQL statement. Some are used to improve the performance of the database, to ensure that the database can be recovered in the event of a software or hardware error, or to perform other tasks necessary to maintain the database. The Oracle server consists of an Oracle instance and an Oracle database.

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Before a database is created, the UNIX environment must be configured and the Oracle9i server must have already been installed. Four environment variables must be set: ORACLE_HOME, ORACLE_SID, PATH, LD_LIBRARY_PATH. ORACLE_HOME is the full path to the top directory in which the Oracle9i Server is installed. The directory for ORACLE_HOME should be supplied by the person who installed the server, usually the UNIX administrator or the DBA. ORACLE_SID is a user-definable name assigned to an instance of a database.

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After completing this appendix, you should be able to do the following: Describe the Oracle Server architecture and its main components List the structures involved in connecting a user to an Oracle instance List the stages in processing: Queries DML statements Commits

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Overview The Oracle server uses many memory components, background processes, and file structures for its backup and recovery mechanism. This lesson reviews the concepts presented in the Oracle9i Database Administration Fundamentals I course, with an emphasis on backup and recovery requirements. Oracle Instance An Oracle instance consists of memory areas (mainly System Global Area [SGA]) and background processes, namely PMON, SMON, DBWn, LGWR, and CKPT. An instance is created during the nomount stage of the database startup after the parameter file has been read.

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Oracle Server Configuration Options Oracle creates server processes to handle the requests of user processes connected to an instance. A server process can be either a dedicated server process, where one server process services only one user process, or it can be a shared server process, where a server process can service multiple user processes. Shared server processes are a part of Oracle Shared Server architecture.

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