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Can someone describe in 30 words what ARCHIVEMODE is for?

I found this: 'When you enable this mode redo logs will be archived instead of overwritten.'

but have no idea, what can I write more about it.

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This answer might help, and will point you to related docs. –  Alex Poole Jun 10 '12 at 19:14
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If a database is in log archive mode, the database makes sure that online redo logs are not overwritten before they have been archived to a designated location. Typically there are more than two online redo logs in rotation within a database environment. If you have a DB file corruption and the transaction is not in your online redo logs, you lose that data. If you have your database in archive mode, then you may still recover using these offline (archived logs).

Archive logging is essential for production databases where the loss of any data might be fatal. It is generally considered unnecessary in development and test environments.

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Also, it is most commonly used in combination with a backup system. Once a new backup has been created, the archived log file is dropped. Otherwise, if the archive log is not set up to rotate, it will keep on growing. The real added value here is that if the database breaks down somewhere past a backup, it can be restored up to the latest point in time by using the archive log files (wich are of course stored on a different machine, for that reason). –  Wouter Jun 11 '12 at 13:08
    
I'm not sure if it will realy be of use if your DB files become corrupted. Maybe in some cases, but i'm pretty shure you will need your backup if your database files become useless. –  Wouter Jun 11 '12 at 13:12
    
If your db becomes useless you can take your most recent good backup and apply the archive redo logs to roll forward from that backup to the most current transaction stored in the newest of the archived redo logs you have available. –  Ram Jun 11 '12 at 20:46
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