Guidelines for Control Files

Provide Filenames for the Control Files

You specify control file names using the CONTROL_FILES initialization parameter in the database initialization parameter file (see "Creating Initial Control Files"). The instance recognizes and opens all the listed file during startup, and the instance writes to and maintains all listed control files during database operation.

If you do not specify files for CONTROL_FILES before database creation:

• If you are not using Oracle-managed files, then the database creates a control file and uses a default filename. The default name is operating system specific.

• If you are using Oracle-managed files, then the initialization parameters you set to enable that feature determine the name and location of the control files, as described in Chapter 15, "Using Oracle-Managed Files".

• If you are using Automatic Storage Management, you can place incomplete ASM filenames in the DB_CREATE_FILE_DEST and DB_RECOVERY_FILE_DEST initialization parameters. ASM then automatically creates control files in the appropriate places. See the sections "About ASM Filenames" and "Creating a Database That Uses ASM" in Oracle Database Storage Administrator's Guide for more information.

Multiplex Control Files on Different Disks

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

The methods for backing up control files are discussed in "Backing Up Control Files".

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.