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Lesson 2Protecting control files
ObjectiveProperly place and multiplex control files.

Protecting Control Files

The control file is one of the most critical files in an Oracle database. It is what Oracle uses to keep track of all the other files in the database. You cannot open your database without it. The control file is also used to record checkpoint and archive log information, which makes the control file critical to the recovery process. Clearly, you can't ever afford to lose your database control files. You can take two approaches to protect yourself from losing your database control file: (1) You can back it up, and (2) you can multiplex it. However, you should do both. Every time you make a change that affects your database's physical file structure, you should back up the control file. Multiplexing the control file means having Oracle maintain multiple copies on separate drives. That way, losing one drive doesn't cause you to lose your control file. The following Slide Show shows how multiplexing your control files protects you from losing a control file because of a disk failure:
1) The buyer (consumer) buys the digital equivalent of money from
2) A digitized check is encrypted (we will discuss security in a later
3) A consumer enters credit card information on a Web order form

Multiplexing Control Files

When you multiplex control files, Oracle keeps the files in synch with each other by always writing the same information to each file. This entails a small amount of overhead as opposed to writing just one control file because everything gets written multiple times, but it's the price you pay for reliability.
When reading, however, Oracle reads only from one control file. How does Oracle choose which file to read?
There's a simple rule. Oracle always reads from the first control file listed in the control_files entry in the database parameter file.
Always multiplex your control files, maintaining at least two copies on separate disks. The next several lessons will show you what you need to know to create more control files for a database, move them around, and delete them if necessary.
In the next lesson, you will learn how to remove a control file from a database.