Overview of Control Files
The database control file is a small binary file associated with only one database. Each database has one unique control file, although it may maintain identical copies of it.
Use of Control Files
The control file is the root file that Oracle Database uses to find database files and to manage the state of the database generally. A control file contains information such as the following:
■The database name and database unique identifier (DBID)
■The time stamp of database creation
■Information about data files, online redo log files, and archived redo log files
■Table space information
The control file serves the following purposes:
■It contains information about data files, online redo log files, and so on that are required to open the database. The control file tracks structural changes to the database. For example, when an administrator adds, renames, or drops a data file or online redo log file, the database updates the control file to reflect this change.
■It contains metadata that must be accessible when the database is not open.
Multiple Control Files
Oracle Database enables multiple, identical control files to be open concurrently and written for the same database. By multiplexing a control file on different disks, the database can achieve redundancy and thereby avoid a single point of failure. If a control file becomes unusable, then the database instance fails when it attempts to access the damaged control file. When other current control file copies exist, the database can be remounted and opened without media recovery. If all control files of a database are lost, however, then the instance fails and media recovery is required. Media recovery is not straightforward if an older backup of a control file must be used because a current copy is not available.