Managing Control Files

Every Oracle Database should have at least two control files, each stored on a different physical disk. If a control file is damaged due to a disk failure, the associated instance must be shut down. Once the disk drive is repaired, the damaged control file can be restored using the intact copy of the control file from the other disk and the instance can be restarted. In this case, no media recovery is required.

The behavior of multiplexed control files is this:
- The database writes to all filenames listed for the initialization parameter CONTROL_FILES in the database initialization parameter file.
- The database reads only the first file listed in the CONTROL_FILES parameter during database operation.

If any of the control files become unavailable during database operation, the instance becomes inoperable and should be aborted.

Note: Oracle strongly recommends that your database has a minimum of two control files and that they are located on separate physical disks.

One way to multiplex control files is to store a control file copy on every disk drive that stores members of redo log groups, if the redo log is multiplexed. By storing control files in these locations, you minimize the risk that all control files and all groups of the redo log will be lost in a single disk failure.


Back Up Control Files

It is very important that you back up your control files. This is true initially, and every time you change the physical structure of your database. Such structural changes include:

- Adding, dropping, or renaming datafiles
- Adding or dropping a tablespace, or altering the read/write state of the tablespace
- Adding or dropping redo log files or groups


Manage the Size of Control Files

The main determinants of the size of a control file are the values set for the MAXDATAFILES, MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, and MAXINSTANCES parameters in the CREATE DATABASE statement that created the associated database. Increasing the values of these parameters increases the size of a control file of the associated database.