RMAN是不備份online redo log的。這個問題一直比較困惑，難道online redo log不需要備份？確實是的

看下面這段

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.

ARCHIVELOG mode時，rman backup中會發出alter system switch logfile 的命令，并備份archivelog ,等同于備份了online redo。
而noARCHIVELOG mode時，不能online backup,所以，redo log是沒有用的。

另外，如果你備份online redo log，并在恢復時使用，覆蓋現有的online redo log，可能會引起數據丟失。

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