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Lesson 8Redo Logs
ObjectiveExplain how Oracle redo logs provide crash recovery

Why redo logs are needed

Another type of file that you will find as part of your database is the redo log file. Redo logs enable Oracle software to recover from a server crash, and they allow for up-to-the-minute recovery if a database needs to be restored from a backup. Whenever you issue a transaction that changes data in a database, the change is written both to the datafiles and to the redo log files, but not neccessarilly at the same time. Look through the following SlideShow to see how this happens:


Redo log file process

How redo logs work

As you can see, Oracle does not immediately write changes to the datafiles .
Find out why.
However, when you commit a transaction, Oracle does guarantee that the changes for that transaction are written to the redo log, and that the commit or COMMIT has been recorded as well. If the server crashes, any changes in memory are lost. However, changes written to the redo log are not lost, because they are already on disk. When you restart the Oracle software, it checks the redo log, and reapplies (or redoes) to datafiles any committed transactions that were lost during the crash.
Click on the following link to read about the Redo Logs .