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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.



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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