教你如何成为Oracle 10g OCP - 第十三章补充：RMAN为什么不备份online redo

RMAN能够备份整个数据库或数据库部件，如表空间、数据文件、控制文件、归档
文件以及Spfile参数文件。这里可能有人要问，为什么那么重要的online redo
log文件不做备份呢 ？

没有必要，ARCHIVELOG mode时，rman backup中一般在备份的时候会执行sql
'alter system archive log current' ， 并备份archivelog ,  相当于备份了
online redo log, 而 NOARCHIVELOG mode时，不能online backup,  所以备份
online redo log是没有必要的。  而且可以想一想，在线备份的时候，一直不
停地在产生新的redo log, 不可能一直备份 ； 还有如果利用备份的online
redo log不小心restore了，可能会导致数据库中的数据丢失 。

 

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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.

 

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