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Donald K. Burleson

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Oracle Redo Log files

As their name implies, redo logs are used to restore transactions after a system crash or other system failure. The redo logs store data about transactions that alter database information. According to Oracle, each database should have at least two groups of two logs each on separate physical non-RAID5 drives, if no archive logging is taking place, and three or more groups when archive logging is in effect. These are relatively active files, and, if made unavailable, the database cannot function. They can be placed anywhere except in the same location as the archive logs. Archive logs are archive copies of filled redo logs and are used for point-in-time recovery from a major disk or system failure. Since they are backups of the redo logs, it would not be logical to place the redo logs and archives in the same physical location. The size of the redo logs will determine how much data is lost as a result of a disaster affecting the database. I have found three sets of multiplexed logs to be the absolute minimum to prevent checkpoint problems and other redo-related wait conditions; under archive log, the use of three groups is a requirement.

Oracle Control Files

An Oracle database cannot be started without at least one control file. The control file contains data on system structures, log status, transaction numbers, and other important information about the database. When tools such as Oracle’s RMAN backup tool are used, the control file can be several tens of megabytes in size. It is wise to have at least two copies of your control file on different disks, three for OFA compliance. Oracle will maintain them as mirror images of each other. This ensures that loss of a single control file will not knock your database out of the water. You should not bring a control file back from a backup, even though this is supported; it is a living file that corresponds to current database status. In both Oracle8i and Oracle, there is a CREATE CONTROL FILE command that allows recovery from loss of a control file. However, you must have detailed knowledge of your database to use it properly. There is also an ALTER DATABASE BACKUP CONTROL FILE TO TRACE command that creates a script to rebuild your control file. In Chapter 15, Backup and Recovery Procedures for Oracle, the section titled “Backup and Recovery Plans for Oracle,” explains in detail how to protect yourself from the loss of a control file. It is easier to maintain extra control file copies.

This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.




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