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Re: Question about Archivemode

From: Howard J. Rogers <>
Date: Thu, 18 Jul 2002 08:29:44 +1000
Message-ID: <ah4r3k$ecu$>

"Ryan" <> wrote in message news:R4lZ8.92701$
> Ok this is a really basic Backup and Recovery question.
> If someone drops the system tablespace, ie, an intruder.
> Is it possible to perform a backup from the redo logs

I'll stop you right there, and say that the question as posed doesn't make sense. You don't backup anything 'from' the redo logs. You use redo logs to recover a copy (ie, a backup) of a datafile.

>in you are not in
> archive mode or have any software like RMAN installed? Or do I have to
> recover from my last cold backup?

The rules are exactly the same, whatever tablespace you are talking about (with the exception of temporary tablespaces). SYSTEM is no different from DATA or TOOLS in this respect. Either you can apply an uninterrupted stream of redo from the time a backup of the datafile was taken to the time the rest of the database is at, or you can't.

If you're not in archivelog mode, then the chances of you being able to supply such an uninterrupted redo stream is vanishingly small. Therefore, you won't be able to bring the backed-up datafile into synch. with the rest of the database, and therefore the database can't be made to function properly.

How then do you get the database consistent and useable? By restoring the entire database from your last cold backup, as you say.

There is one scenario where, despite not being in archivelog mode, you might get away with a complete and perfect recovery: if the continuous stream of redo you require still happens to be sitting in the online logs. If you have 5 online redo log groups, which switch every half hour, then you have 2.5 hours of redo available to you. So if you backed up last night, opened the database at 9.00am, and the intruder swiped the system tablespace at 11.00am, you're doomed. But if he swiped it at 9.45am, the redo required to recover the backup of the system datafile is still available in the online logs, so you could just restore that one file and do a recovery on it.

Clearly, there's no guarantees, though, that the required redo would be available. Only archivelog mode can make that guarantee.

> For those of you who saw an earlier post of mine... no this did not
> :)
Received on Wed Jul 17 2002 - 17:29:44 CDT

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