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Re: Minimizing backup induced downtime

From: Alexander Skwar <>
Date: Thu, 12 Jul 2007 23:23:49 +0200
Message-ID: <>

ยท <>:

> Alex:

>>Suppose you're doing a RMAN backup at 22:30. At 22:45 it's
>>done, ie. also writing to tape is finished at that time.
>>At 23:00, the server dies and the tapes are still intact.
>>With that magical RMAN thing, how will you be able to recover
>>anything that's changed in the database after 22:46? My, maybe
>>ncomplete, understanding of how RMAN works, is, that it can
>>take a backup of an Oracle database without the need for having
>>it shutdown (thus a hot backup). But how will RMAN allow me
>>to recover something, which hasn't been backed up?

> I've done just such a test and it works magically. I'm beginning to
> think that you either aren't running in archive log mode, or you don't
> know what that is.

I am running the databases in archive log mode and I know what it is. But the problem is, that the logs aren't backed up. They are, at best, backed up until 22:45. Then the backup is finished. Everything that's stored on the disks/in the database between 22:46 and 22:59 has not been backed up yet.

It's possible that I don't understand something here. But how can you recover the database, so that it is in the state it was at 22:55? Remember, the archive log files are lost. I'm really interested - how can that be done? I mean, on tape (or on some other secure location/medium) there's the database the way it was up until 22:45, at best. Isn't it? From where does RMAN/Oracle pull the data about what has been done between 22:46 and 22:59, if archive logs aren't available?

> Sooner or later your luck will run out.

Hm. Not impossible. ACK.

> What if the server dies
> during your ZFS snapshot or your export? Then you have to go back
> even farther?

But isn't it the same with RMAN or "old school hotbackups running in, archive redo log mode"? Suppose you're doing a copy to a secure medium (tape/off site disk/whatever) only every 24hrs - how would RMAN/archive redo log be an improvement in this case?

> I posted some helpful info in the other thread on places to look for
> help with RMAN (sample RMAN scripts in $ORACLE_HOME/rdbms/demo) but
> you clearly did not see that.

Nope. I have seen that and sent you an email reg. this about 2hrs ago. I did not yet read the ressources you pointed me to. I'll do that tomorrow morning.

> turn 40." It's not the age, it's the unwillingness to learn and apply
> the better technology where it's appropriate that wrecks careers.

Exactly. Where it's appropriate. As it is, I need to backup Oracle and some other directory. Those two need to be in sync. And here comes another reason against RMAN or even archived redo logs: As I wrote, the two directories need to be in sync. I take a backup of the directory and also Oracle every 24 hours. Suppose that the storage on tape is good (which is an entire different question...). What good would it do me in real life, if I could restore/recover the Oracle database to a PIT (point-in-time) it was 18 hours ago, when I cannot do the same to the directory? Yes, I know that there are setups, which would allow me to do even more snapshots, like NetApp filers. But we don't have that.

It's all nice, that I can do a PIT of Oracle - but really, what do I do with that? Not in general or in a Oracle only world, but in the scenario I described.

Alexander Skwar

It was a JOKE!!  Get it??  I was receiving messages from DAVID LETTERMAN!!
Received on Thu Jul 12 2007 - 16:23:49 CDT

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