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Re: database could not be started

From: quarkman <quarkman_at_myrealbox.com>
Date: Tue, 12 Aug 2003 16:38:03 +1000
Message-ID: <oprtr4ppeozkogxn@haydn>


On Mon, 11 Aug 2003 20:47:37 -0500, Burton Peltier <burttemp1REMOVE_THIS_at_bellsouth.net> wrote:

First off, you're still on about backup temperature, and that's got nothing to do with it. You can do cold backups in archivelog mode.

Second, I was not advocating blindly doing open resetlogs, and thus blowing away the contents of the online logs.

I'll try and make this simple.

Noarchivelog means any recovery can only ever be to the last clean, whole database backup (barring one or two quirks). Therefore, you will lose all data entered since that last backup. Therefore, surviving online redo logs are useless anyway. Therefore, you don't need them. An open resetlogs will blow them away and re-create them. Have you lost anything you wouldn't otherwise have lost? No. Does this mean I would do resetlogs with gay abandon whenever the mood toook me? No, of course not, because in archivelog mode, a resetlogs is a very serious business indeed. But in noarchivelog mode, it's trivial.

Alternative scenario: noarchivelog, and your complete backup includes redo logs. Recovery is simply to restore everything. You lose every transaction entered since the time of the last backup (ie, exactly the same as in the previous scenario). You avoid a resetlogs, but as I say that's really only an issue when you're in archivelog mode anyway. Costs? Depends on the size of your redo logs. But restoring 3 or 4 500Mb files when you didn't actually gain anything by doing so, and wouldn't have lost anything by not doing so, seems to me to be a waste of time.

Next scenario: you're in archivelog mode. Recovery consists of restoring only the faulty data file, and then rolling all the way forward, using as you do so, the transactions in the current online redo log. Did you need the online logs to be included in the backup? No, because you only need to restore the faulty data files.

Final scenario: archivelog mode, but you take copies of your online redo logs. A couple of data files go wrong... your junior DBA tries to take a short cut, and restores the entire last night's backup. You've just over- written the fresh online logs with two very stale ones. You therefore can't roll completely forward, and you've lost data as a result (at least all the transactions that were in the current log). Do you regret backing the logs up now? Yes you do, because you've just lost data in a scenario that should have been completely recoverable.

Point is: it's dangerous to back up the logs in archivelog mode. Mistakes happen, and they're never needed anyway. It's pointless backing them up in noarchivelog mode, because you don't need them in order to be able to recover as completely as its ever possible to recover in noarchivelog mode anyway.

But, as has been pointed out, a dirty shutdown prior to backup would mean the current redo log would be needed to achieve a consistent (and openable) database. Therefore, don't do dirty shutdowns.

I hope that clarifies what I was trying to say, anyway!

Regards
HJR Received on Tue Aug 12 2003 - 01:38:03 CDT

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