What is multiplexing of a control file?

What is multiplexing of a control file?

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Keeping multiple copies of a control file in a database is known as multiplexing of the control file. Multiplexing a control file and keeping each copy on a separate disk reduces the risk of losing all control files in case of a single disk failure. A database can have a maximum of eight control files. During database operations, Oracle writes the same information to all control files listed in the CONTROL_FILES parameter. However, it reads only the first control file. If any of the control files becomes unavailable during database operations, the database becomes inoperable and must be aborted.

A control file can be created and multiplexed at the time of database creation by specifying the names and locations of the control file and its copies in the CONTROL_FILES parameter of the database's initialization parameter file.

A control file can also be multiplexed after database creation by editing the initialization parameter file and specifying the names and locations of its copies in the CONTROL_FILES parameter. If the initialization parameter file is a binary file (server parameter file), it can be edited using the ALTER SYSTEM statement. If the initialization parameter file is a text file, it can be edited manually. After editing the initialization parameter file, the control file must be manually copied to each location specified in the CONTROL_FILES parameter. While copying the control file, the database must be closed. The changes come into effect when the database is re-started.

When starting an Oracle database, all the control files listed in the CONTROL_FILES parameter must be available. Otherwise, the Oracle instance starts, but it fails to mount and open the database. This is because Oracle cannot mount a database without reading a control file.

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