教你如何成為Oracle 10g OCP - 第十三章補充：RMAN為什麼不備份online redo

RMAN能夠備份整個資料庫或資料庫部件，如表空間、資料檔案、控制檔案、歸檔
檔案以及Spfile引數檔案。這裡可能有人要問，為什麼那麼重要的online redo
log檔案不做備份呢 ？

沒有必要，ARCHIVELOG mode時，rman backup中一般在備份的時候會執行sql
'alter system archive log current' ， 並備份archivelog ,  相當於備份了
online redo log, 而 NOARCHIVELOG mode時，不能online backup,  所以備份
online redo log是沒有必要的。  而且可以想一想，線上備份的時候，一直不
停地在產生新的redo log, 不可能一直備份 ； 還有如果利用備份的online
redo log不小心restore了，可能會導致資料庫中的資料丟失 。

 

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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.

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RMAN does not backup online redo and temp ts, and online redo logs do
NOT need to backup in either cold and hot backup. When it comes to
recovery, NEVER replace your current online redo logs. In case of anything
 wrong with them, clear logfile or resetlogs.