Whether or not backing up online redo logs confuses me. RMAN itself doesn't backup online redo log files at all. However, suppose you have a clean shutdown and a clean cold backup (except redo logs) and now you are about to open your database, you find out that you lost all your redo logs, can you still open your database?
As long as you have all your datafiles and control files from a valid cold backup, you can open your database with the resetlogs if you don't have the log files. However since resetlogs is only valid after a recovery, you have to fake one.
svrmgrl> startup mount
svrmgrl> alter database recover until cancel; -- says statement processed
svrmgrl> alter database recover cancel; -- says recovery hasn't started
svrmgrl> alter database open resetlogs;
As long as it's a valid cold backup, and you don't need any data from online logs or archived logs, you won't lose anything.
You need all the files I mentioned in previous posting if your database is in no archive mode and shutdown properly.
You need archivelog files also, if your database is in archivelog mode,as your database checks for SCN sync when you open the dataabse. If your logs are not in sync you can't open the database in archivelog mode.
In such situations, you open the database resetting the logs and start archiving freshly which gives a begining SCN.
No, you could fake it by issuing 'alter system switch logfile' as many times as you have groups (assuming nothing else is going on inside the database), but it doesn't do it automatically.
When you take a cold backup, all the data is commited into the data files and you don't need the data that is in the redo log files. This is why the redo log files aren't NECESSARY for a cold backup, but it's is common practice and easier just to group them with the cold backup.
However if you take a hot backup, and the database crashes sometime after, you need all the information from the archived redo log files associated with the backup, and the current online redo log files to recover fully.