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Database system files

 Data files

Control files: control files are used to save the database physical structure this includes the database file names and their locations the backup files and the database basic information like creation time and last check

 Oracle database instance needs only a single control file to operate. However in most production data bases more than one control files are configured no Oracle database can operate without control files 

 Control files must be there otherwise the instance goes down.

Parameter file: the parameter file is a small file to save the parameter value pairs. The parameter is saved in the file are called installization parameters.

 Installization parameters are configuration parameters that controls the instance behaviour.

Online redolog files: online redo log files are two or more pre allocated files that store changes to the database as they occur.

 The files are used for instance recovery, generating archive redolog files, replication and data guard.

System data files: system data files are data files which contain data used by Oracle internal operations. 

 For example Windows folder in Windows platform are system data files.

 Windows folder contains a set of files that are used solely by Windows operating system. The user should not save their own files in the Windows folder. 

 Going with this analogue as Windows cannot operate if it's Windows folder is gone Oracle database cannot operate if it system data files are deleted. They are mandatory for normal database operation.

Automatic Diagnostic Repository (ADR): ADR is a simple directory structure where the database saves data such as trace files alert log files, DB log files and health monitoring reports.






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