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Home -> Community -> Usenet -> c.d.o.server -> Re: Online backup: Backup online redologs?
Howard J. Rogers wrote:
>>>>> See above. It makes no sense to backup the online redo logs. They've >>>>> already all been backed up to the archives, with the exception of the >>>>> current log. If you are that concerned, then a switch log file will >>>>> cause >>>>> that to be archived, and by backing up that last archive, you've got >>>>> the >>>>> lot. >> EXCEPT that you cannot execute the output of 'ALTER DATABASE>> BACKUP CONTROLFILE TO TRACE' without the presence of the >> redolog files, one of which must be marked as active.
> You shouldn't take a reply to one person's query out of context. That reply
> was referring to a worry about not having the very last transactions in a
> given hot backup, and had nothing to do with cloning a database.
Then why was it part of your reply?
I also shouldn't take hot backups of the redo logs, but that seems to work pretty well, doesn't it? The emphasis here stressed the discontinuity of your reply.
>> What is really the difference between a restore with a hot >> copy of the redo and a recovery of a crashed instance which >> corrupts a number of tablespaces? >> Wouldn't each situation >> introduce the same sort of corruption?
> Yes, sort of. Both sorts of files will be internally inconsistent (corrupt,
> if you prefer). Corruption is corruption is corruption. It's what you can
> *do* about the corruption that makes the difference, and there's sod-all you
> can do about corruption in redo logs.
Either it is the same or it is different; you must choose.
>> It would make much more sense if there were an option to the >> CREATE CONTROLFILE statement to create blank redologs. Oracle >> should struggle to make this easy; this ambiguity is not useful.
> Where's the ambiguity? "You cannot take hot copies of redo logs" is pretty
> unambiguous. And "mirroring (or multiplexing iof you prefer) was invented
> precisely so that you would never be in a position of losing all members of
> a redo log group" is likewise fairly clear-cut.
In a cloning situation, *I only want to apply the redo that is in the archived logs.* I could care less about the online redo - really, absolutely, I don't want it, and I don't want to have to worry about it.
Actually, I feel mostly the same about backing up to tape. I just don't care about unarchived redo.
But, as I said, *YOU CANNOT CREATE A NEW SET OF CONTROLFILES WITHOUT THE REDOLOGS*, which implies to me that cloning from a set of hotbackups is not supported with a controlfile trace. If this is so, then what is the point of hotbackups? It might as well be Microsoft Notepad with a SQL interface. Oracle's tremendous flexibility with datafile manipulation is negated by the lack of proper management of this single component.
Whether I am cloning or backing up to tape, I want to hold in my hand all required components to COMPLETELY restore the database. 99% is not satisfactory.
>> I don't understand why people are so evasive on this issue.
> I don't understand why certain people can't simply approach this issue with
> logic and clarity, instead of seeking to do the impossible by utterly
> illogical means.
Yes, at this point, it appears that cloning from hot backups using a controlfile trace is impossible FROM AN ORACLE SUPPORT PERSPECTIVE, nevermind that it is demonstrated to work in practice in at least certain situations (actually, in every situation that I've seen).
The third Oracle support technician that I spoke with slyly implied that this method would never cause trouble, so I've heard answers all over the map on this question, most of them wrong (as I previously demonstrated). Pardon me for my scepticisim.
Honestly, I don't understand why this process isn't better addressed in the documentation and better understood by support personnel. What is the purpose of a hotbackup if you can't restore it?
In any case, I don't want to argue; let's try a lateral approach. Assuming that I have a backup controlfile produced by "ALTER DATABASE BACKUP CONTROLFILE TO '/CONTROL.BAK';", what is the procedure to recreate a set of online redologs (including an active thread)? Of course, I would rather not do it this way - the controlfile trace will allow me to make many changes to the positioning of the datafiles in one go, including the system tablespace. But assuming that I am willing to endure the lesser quality, how are the redologs recreated? Received on Tue May 01 2001 - 09:18:16 CDT
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