Offline or Online Backup after switching off and on the archving mode

Question

Hello guys,
I am doing an SAP (on Oracle) upgrade. In the upgrade guide there are two completely different statements about making a backup of the Oracle database during upgrade.
1st statement:
"If you deactivate archiving, you have to make a full backup of the database after the downtime before you switch archiving back on. This must be executed as an offline backup."
2nd statement:
"Backup: Database Was Operated in NOARCHIVELOG Mode
Activate ARCHIVELOG mode again and create a full backup of the database. This must be executed as an offline backup."
Quotes from: "Upgrade GuideSAP ERP 6.0 Including Enhancement Package 4 Support Release 1 ABAPBased on SAP NetWeaver 7.0 Including Enhancement Package"
I understand the fact that the backup has to be offline if you do it before you activate archiving. This is logical.
1) Why does SAP do these two proposals before and after switching on archiving which are completely contrary?
2) Why does the backup has to be offline if you do it after switching on archiving?
May be somebody knows the background for this.
Thanks
Nik

Answer 1

Hi,
Two kind of Recovery play an important role in Database Recovery after DB Restore. The Database recovery can be Compete recovery, point-in-time recovery or partial recovery.
Media Recovery (due to media failure/corruption or loss of Data files) can be performed by applying all Offline Redo Log files which are generated from Online Redo-log file in Archive Mode.
Instance Recovery can be done with the help of Online Redo log file and undo table-space to achieve consistent status of Database. In this recovery the committed transactions (which are logged in Online Redo Log files) are roll forward and the uncomplete transactions (which are also logged in Online Redo Log Files) are rolled back into DB using Undo space info.
If you deactivate archiving, you have to make a full backup of the database after
the downtime before you switch archiving back on. This must be executed as an offline backup.
                   With Archive log Mode , one can deal with Media Recovery and Instance Recovery. Upon deactivating Archive log mode, the Archive log files will not be generated and the circular logging will take place within the group of online redo log files.
                   So, after deactivating Archive Log mode, Its recommended to take immediate offline backup to have consistent Backup copy of Database which may be required to deal with demanded media recovery by restoring that immediate latest offline Backup. If the database operates in NOARCHIVELOG mode, the database can ONLY be completely recovered from instance failure but not from media failure. This means that, in case of media failure you will lose all work done after the last valid offline backup. The database can be backed up only while it is completely closed.
                   Upon media failure after deactivating the Archive log mode (..rare chance, but we can not ignore it...),the complete recovery can not be achieved if the large time gape is there between the last successful offline backup and DB crash because between this time it may be the chance that the Online Redo log files are overwritten so many times in circular manner.