taping oracle online redo logs?

[Alexander M. Ermes]

Hi Michael.
Why not using archivelog. store the offline logs on different disks, compress them every twenty minutes and put them on the backup as well ?
I have attached a script for compressing offline redo logs.
Other idea is to do a fll database backup once a week and backup the offline redo logs every day.
Just my 0.02 â ¬
Rgds
Alexander M. Er

[Michael Schulte zur Sur]

Hi Nelson,

thank you for your sympathy. However I am talking about saving the redolog before it is finished. Once the log switch succeded, yous should be safe.

Hi Alexander,

thanks for the script. We will of course be taking any ordinary measure to secure the data. Depending on the speed of the disk subsystem, a copy could be done in a blink of an eye, without forcing to many log switches onto Oracle.

thanks

Michael

[Henrik Palmqvist]

Hi Michael,

"The certainty, you make the statement, that Oracle canà ´t do anything with a redolog, that is copied, while Oracle is running, makes me think, you are one of the programmers at Oracle, who wrote that section of code ;-)."

Damn... I knew I wouldn't be able to mask my true identity for long in this forum... ;-)
No, I do not work for Oracle.

I'm not saying that Oracle tech personell can't do anything with a copied online redo log, I'm just saying that it is not a supported backup method.


"I posted the question at a metalink forum and not even an Oracle export said so, as you do."

He did actually.
Mark Bobak from Oracle says:
* "Backing up online redo logs is never necessary."
* "In case of hot backup, Oracle provides no mechanism to make a clean, consistent copy [of the online redo log]."
* "There are really no recovery scenarios where you'd need a backed up copy of online redo."

Which is not so different from what I and others in this thread has stated earli

[Michael Schulte zur Sur]

Hi Henrik,

thanks for your answer. I was always unsure, whether it could work or not, but since this is a complicated subject, not many can say for sure, whether it works or not, because "not supported" does not allways mean "not working" ;-). As I said before, we will have to discuss alternatives for a tighter data safety.

thanks for everyone,

Michael

[John Palmer]

Hi Michael,

You are proposing a very dodgy strategy.

That quote from Oracle says it all:
'Oracle provides no mechanism to make a clean, consistent copy [of the online redo log]'

How can you be sure that the copy you have taken is in any way consistent? Oracle writes to these logs in a disciplined fashion and ensures that the updates are actually on disk before committing a transaction. This is contrary to (for instance) datafile updates which may only be updated in the UNIX buffer cache and may take some considerable time to be flushed to disk. This strategy is crucial to the way that Oracle manages recovery. Your copy method would have to work in a similar way, cp or dd for instance would use the buffer cache.

The ONLY way that you can do what you want is as suggested earlier to get Oracle to switch the logfile (you've been given good advice).

The scenario that you are trying to guard against is one of total disaster - all datafiles and redo log copies lost. If you implement a strategy of switching redo logs every 10 minutes say and ftp'ing them to an offsite server then you have the ability to recover to within 10 minutes of the disaster. It's going to take you quite a while to do the recovery so I'd say that that wasn't bad in the circumstances.

Regards,
John