Hi
An offline copy of an online redo logs is called an archived redo log. You must run the database in ARCHIVELOG mode to archive redo logs
Generally online redo logs are of fixed size. typically 50 MB (We can change this value through parameters)
Offline redo log files are used in system recovery. Especially, when you want to perform point-in-time recovery. When we set Archive logging option to 'on' in database; offline redologs will be generated. This will ensure the system recovery. Normally for a production system we will mirror these offline redo-logs and then store it on tapes or some other disk. FYI...I have pasted below information.
Importance of Redo Log Files
When you want to perform a complete recovery, to recover the data to the (committed)
state the data had at the moment of crash, you need all the offline and online redo log
files written from the point in time of the last database backup.
If a file is missing from the chain of offline redo log files, a restore of subsequent
offline redo log files and corresponding recovery of the database is not possible. You
can then only perform a point-in-time recovery, using all offline redo log files older
than the lost one. This will result in a loss of data changes performed from the point in
time of your lost offline redo log file. Therefore, you should keep at least two copies
of all offline redo log files on disks or on tapes.
Cheers!
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