Re: Online backup: Backup online redologs?

From: Charles Fisher <Charles.Fisher_at_alcoa.com>
Date: Tue, 15 May 2001 14:37:21 GMT
Message-ID: <Pine.GSO.4.31.0105150914480.1016-100000@unknown>

On Sun, 13 May 2001, Howard J. Rogers wrote:

> Redo Logs: well, one (or more) of them is dead. Nothing is being written
> there, so if we hot copy it, the results will be consistent. But one of
> them is hot (the current log), and if we start copying that as it is being
> written to... well, I'll lay odds the results will be a mess. Do we have
> something to apply to the current redo log copy to make it sensible. No we
> do not... you can't apply redo to the redo logs.

> So... the net result is, we cannot copy the current redo log whilst it is
> hot, because the resulting output will likely be a pile of nonsense, and
> nothing can put it right. And forget the stuff Charles has been posting
> about managing to do it: he"s merely *got away with it*, and for a very
> simple reason: he makes sure his database is fairly quiescent first, so that
> his current redo log is unlikely to be written to. The absolute way of
> ensuring the current redo log is safe to copy is, of course, to shut the
> database down completely: now it's cold, and of course the copy results will
> be consistent. That's why cold backups invariably include the redo logs.

Howard, I suggest that you examine oraback.sh at backupcentral.com.

From their description:

"If it's a hot backup...

The script then backs up the control files to disk using both svrmgr and a manual copy. The manual copy is just for the paranoid; it's not a supported method of backing up the control file. Finally, the script makes copies of the essential redo logs and compresses them. (Essential logs are those that span the time of the backup.) If the backup device is a tape, it then backs up all files that were sent to disk to the tape device."

They do go on to say:

If it's a cold backup...

The script then backs up the control files and redo logs the same way it does during a hot backup. This is perhaps the only time that manually copying the control files and online redo logs does any good.

From a recent mailing:

There's several thousand of you using Backup Central now, and we'd really like to spend our limited time developing features that you actually want!

I'm not saying that I agree with everything they are doing (I don't see why anyone would use an OS copy of the controlfile).

However, with such a large userbase and the interaction of so many DBAs in the production of the script, your opinion on the merit of hot redolog backups is in question. Go and do some research.

p.s. You are also wrong in saying "The absolute way of ensuring the

     current redo log is safe to copy is, of course, to shut the
     database down completely: now it's cold, and of course the copy
     results will be consistent." - If the database was brought down
     with a shutdown abort, the active online redolog will NOT be
     consistent.

-------------------------------------------------------------------------------

Charles J. Fisher - Consultant
Alcoa Davenport Works
(319) 459-2512 Received on Tue May 15 2001 - 09:37:21 CDT