Oracle-L: RE: Confusion why online redo logs should never be backed up.

From: David Wagoner <dwagoner_at_arsenaldigital.com>
Date: Thu, 22 May 2003 11:22:44 -0800
Message-ID: <F001.005A0639.20030522112244@fatcity.com>

Excerpt from the 9iR2 Backup and Recovery Concepts doc:

----------------------begin excerpt;

Avoiding the Backup of Online Redo Logs
Although it may seem that you should back up online redo logs along with the datafiles and control file, this technique is dangerous. You should not back up online redo logs for the following reasons:

The best method for protecting the online logs against media failure is by multiplexing them, that is, having multiple log members in each group, on different disks and disk controllers.
If your database is in ARCHIVELOG mode, then the archiver is already archiving the filled redo logs.
If your database is in NOARCHIVELOG mode, then the only type of backups that you should perform are closed, consistent, whole database backups. The files in this type of backup are all consistent and do not need recovery, so the online logs are not needed.
You may accidentally restore backups of online redo logs while not intending to, thereby corrupting the database.
A number of situations are possible in which restoring the online logs cause significant problems to the database. The following sections describe scenarios that illustrate how restoring backed up online logs severely compromises recovery.

Unintentionally Restoring Online Redo Logs: Scenario When a crisis occurs, it is easy to make a simple mistake. When restoring the whole database, you can accidentally restore the online redo logs, thus overwriting the current online logs with the older, useless backups. This action forces you to perform incomplete recovery instead of the intended complete recovery, thereby losing the ability to recover valuable data contained in the overwritten redo logs.

Erroneously Creating Multiple Parallel Redo Log Timelines: Scenario If you face a problem where the best course of action is to restore the database from a consistent backup and not perform any recovery, then you may think it is safe to restore the online logs and thereby avoid opening the database with the RESETLOGS option. The problem is that Oracle eventually generates a log sequence number that was already generated by the database during the previous timeline.

For example, say that the most recent archived log for database prod1 has a log sequence number of 100. Assume that you restore a consistent backup of the database along with backed up online redo logs and then do not open with the RESETLOGS option. Assume also that the restored online log is at log sequence 50. Eventually, the database archives a log with the log sequence number of 100--so you now have two copies of log 100 with completely different contents.

If you then face another disaster and need to restore from this backup and roll forward, then you may find it difficult to identify which log with sequence number 100 is the correct one. If you had reset the logs, then you would have created a new incarnation of the database. You could only apply archived logs created by this new incarnation to this incarnation.

Note:
RMAN does not permit you to back up online redo logs.

---------------------end excerpt;

Best regards,

David B. Wagoner
Database Administrator

-----Original Message-----
Sent: Thursday, May 22, 2003 2:27 PM
To: Multiple recipients of list ORACLE-L

Hi All,

In Oracle8i Backup & Recovery Handbook pg 13 it states "Finally, online redo log files should never be backed up....So, it is good practice not to back up online log files for ANY kind of backup.

Can someone clearly explain why this is recommended?

Thanks
Rick

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Author: David Wagoner
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