Re: Backup Oracle ? Online Redo Log.

You cannot, ever, in a million years, backup an online redo log. It can't be done, and you'll risk data loss by attempting it (I've seen a junior DBA restore everything from a backup set, including a hot-copied online log, and as a result over-write a perfectly functioning current redo log with a pile of poo).

Rogers' first rule of hot backups is simply this: You cannot ever hot backup any part of an Oracle database without the end result being internally inconsistent. That goes for online redo logs as well as datafiles or control files.

Rogers' second rule of hot backups is that internally inconsistent files are not a problem provided Oracle's supplied a mechanism to get them internally consistent. The application of redo makes an internally inconsistent data file consistent. But for control files and redo logs, there is no such mechanism (you can't apply redo to either).

Now, for control files, Oracle thoughtfully provided the 'alter database backup controlfile to...' command, which uses its own special mechanism to generate either a trace file (and there's no worries about a text file being inconsistent) or a guaranteed consistent binary image of the control file.

But no such provision was made for taking a hot backup of the current redo log.

Therefore, combine rules one and two with the lack of an alternative mechanism and the conclusion is inescapable: hot backups of the current redo log will be internally inconsistent, utterly unusable, and no mechanism exists to rectify the situation.... therefore, hot backups of them are a complete waste of time.

You can of course perform a cold backup of said redo log, by the simple expedient of shutting the database down.... at which point, you don't actually have a current redo log any more, just a bunch of dead o/s files. There is no problem copying such a closed redo log at all... but it will be out of date the second you re-open the database.

Visit my site, read the backup and recovery documents there, especially the large one under the 'books' link, and you'll see that unless you are running in noarchivelog, there is zero usefulness in backing up the current log at all. What's more, doing so risks inadvertent (and unnecessary) loss of data when it comes time to recovery. Without doubt, the current redo log is the Achilles' heel of the entire Oracle architecture -which is why they invented redo log mirroring (or, more accurately) multiplexing so that total loss of the current log should be a very rare occurrence -making attempted backups of it even more of a waste of time than they otherwise would be.

Regards
HJR

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"Christian MERCK" <cm_at_sdv.fr> wrote in message
news:3c3d4dda$0$14589$4d4eb98e_at_read.news.fr.uu.net...


> Hi,

>

> It is useful to backup regularly the online redo log ?

> If yes, how should I do it ? Just an OS copy ?

>

> Thanks in advance.

>

> Christian.

>

>