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Re: Problem

From: Howard J. Rogers <howardjr_at_www.com>
Date: Sun, 18 Feb 2001 08:20:07 +1100
Message-ID: <3a8eeb09@news.iprimus.com.au>

"FL" <francis_at_mail.com> wrote in message news:3a8e28bb.4294605_at_news.i-cable.com...
> What is the difference of noarchivelog mode or archivelog mode?

About a bit!

When you issue the command 'alter database archivelog' in the mount stage after a clean shutdown, you are talking to the Controlfile, and setting a bit there which prevents Log Writer (LGWR) from over-writing the contents of an on-line redo log until and unless it has been successfully copied out somewhere. Since you only have 2 (or up to 32) online redo logs, Oracle keeps going ordinarily (and by default) by simply moving through each log at a time, and over-writing the contents of the first after it's finished with the last -and the cycle just keeps going. Setting the archivelog bit stops the cycle in its tracks (and, incidentally, hangs your database) unless you *also* remember to switch on something that will do the copying of used redo logs. That copying is done by the ARCH process, and he's switched on by setting LOG_ARCHIVE_START=TRUE in your init.ora.

To successfully archive online redo logs, therefore, you must do both the 'alter database archivelog' AND the editing of your init.ora.

Why take archives? Because redo is the record of all the transactions you have done on the database. If you have a continuous record, you are guaranteed to be able to perform all of them in the event of a datafile being corrupted or deleted, or an entire hard disk failing. Archives thus guarantee complete data recovery (and give you all sorts of added benefits, like being able to do hot backups, and backups of pieces of the database instead of the entire thing).

If you don't take archives, you can't guarantee that. If some of your transaction stream has been over-written, how can you re-perform those transactions if you need to?

A slightly dodgy analogy I sometimes use is that online redo logs is like sitting down to watch the television. Setting archivelog mode is like switching on the video recorder.

Regards
HJR Received on Sat Feb 17 2001 - 15:20:07 CST

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