Oracle overview

DATABASE

  A collection of information that has been systematically organized In form of physical files for easy access and analysis..

  A database is a set of files: 

                                        1. Control file

                                        2  Redo file.

                                        3. Datafile

                                        4.  Parameter file

                                        

       Every running Oracle database is associated with an Oracle instance. When a database is started on a database server , Oracle allocates a memory area called the System Global Area (SGA) and starts one or more Oracle processes. This combination of the SGA and the Oracle processes is called an Oracle instance. The memory and processes of an instance manage the associated database's data efficiently and serve the one or multiple users of the database.

       PARAMETER FILE (init<sid>.ora)

Oracle uses a parameter file when starting up the database. 

The pfile is a text file containing the parameters and their values 

for configuring the database and instance.

The default location is under $ORACLE_HOMe/dbs directory 

 The parameter files tell oracle the following when starting up an 

instance:-

•The name of the database and location of control files.

•The size of the SGA.

•The location of the dump and trace files.

•The parameters to set limits and that affect capacity.

Some of the important parameters are:

·        db_block_size

·        db_files

·        undo_management

·        log_buffer

·        max_dump_file_size

·        db_block_buffers

·        shared_pool_size

     log_checkpoint_interval

INIT<SID>.SAP

       The SAP utilities BRBACKUP, BRARCHIVE, and BRRESTORE must be configured before they can be used. To do this you must set the appropriate parameters in initialization profile init<DBSID>.sap. Before using one of the SAP utilities, find out exactly which parameters you have to configure. Changes to parameter values do not take effect until you call the corresponding utility.

  InitSID.sap is located in /usr/sap/<SAPSID>/SYS/exe/run/initSID.sap

  Important parameters in init<Sid>.sap

•archive_copy_dir: 

This parameter identifies the directory used by BRARCHIVE to back up the offline redo log files to a local disk 

•backup_mode:  

This parameter is used by BRBACKUP and BRRESTORE to determine the scope of the backup/restore activity.

•backup_type: 

Identifies the default type of the database backup. This parameter is only used by BRBACKUP

  

tape_size: Storage size in gigabytes (G), megabytes (M) or kilobytes (K) for the tapes that will be used for backups and for archiving redo log files.

remote_host: this parameter is to specify the name of the remote host, if you want to make a backup to a remote disk

volume_archive: This parameter is used by BRARCHIVE to identify the  volume/directory to be used for the archive of the offline redo log files

volume_backup: This parameter is used by BRBACKUP to identify the volume/directory to be used for the backup of the database or non-database files

backup_dev_type: Determines the backup medium that you want to use .It may be a disk, tape etc

  Database Buffer Cache:

  Is a fairly large memory object that stores the actual data blocks that are retrieved from datafiles by system queries and other data manipulation language commands

  The buffers in the cache are organized in two lists:

–The write list and,

–The least recently used (LRU) list.

  The write list holds dirty buffers – these are buffers that hold that data that has been modified, but the blocks have not been written back to disk.

  The LRU list holds free buffers, pinned buffers, and dirty buffers that have not yet been moved to the write list.  Free buffers do not contain any useful data and are available for use.  Pinned buffers are currently being accessed

  Redo Log buffer Cache:

       The Redo Log Buffer memory object stores images of all changes made to database blocks.  As you know, database blocks typically store several table rows of organizational data.       This means that if a single column value From one row in a block is changed, the image is stored.  Changes include insert, update, delete, create, alter, or drop.

      Data dictionary Cache:

  The Data Dictionary Cache is a memory structure that caches data dictionary information that has been recently used.  This includes user account information, datafile names, table descriptions, user privileges, and other information.

  The database server manages the size of the Data Dictionary Cache internally and the size depends on the size of the Shared Pool in which the Data Dictionary Cache resides.  If the size is too small, then the data dictionary tables that reside on disk must be queried often for information and this will slow down performance.