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Control files [Oracle]
The control files of a database store the status of the physical structure of the database. The control file is absolutely crucial to database operation. It contains (but is not limited to) the following types of information:
The location of the control files is specified through the control_files init param.
Backing up Controlfiles
When a controlfile (multiplexed or not) becomes unavailable (because for example of a media failure, the database cannot continue to operate.
Because control files keep track of datafiles the control file should be backed up whenever a change is made to the database (alter database).
The command to back the control file up is: alter database backup controlfile to 'filename'.
Recovering database when controlfile is lost
If the controlfile is lost, it is somewhat difficult to do a recovery because the database cannot be mounted for a recovery. The controlfile must be recreated (with create controlfile whose syntax is a bit awkyard.
In order to recover from a lost spfile, the DBID is required.
Size of control files
The size of the control files is governed by the following parameters:
The controlfile contains the following sections:
The minimum number of days that a reusable record is kept in the controlfile is controlled by the control_file_record_keep_time parameter.
These sections consist of records. The size, total number and number of used record are exposed through v$controlfile_record_section.
SQL*Loader's behaviour is specified with the SQL*Loader control file. Although they sound similar, they have nothing in common.
ASM simplifies the optimal layout of controlfiles.