RMAN不备份online redo log

RMAN是不备份online redo log的。这个问题一直比较困惑，难道online redo log不需要备份？确实是的

看下面这段

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时，rman backup中会发出alter system switch logfile 的命令，并备份archivelog ,等同于备份了online redo。
而noARCHIVELOG mode时，不能online backup,所以，redo log是没有用的。

另外，如果你备份online redo log，并在恢复时使用，覆盖现有的online redo log，可能会引起数据丢失。

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