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RE: Question in HOT BACKUP

From: Mark W. Farnham <>
Date: Sat, 2 Oct 2004 09:52:51 -0400
Message-ID: <>

Just a bit more:

  1. The first step of recovery is to get a secure copy of whatever the state is of the current online redo logs. This is before you restart the instance, the files are cold, and it gives you a chance to try again if there is pilot error or a physical problem with the machine during recovery. You do this before you start hanging tapes or "silvering" from an online backup set. If you reach "open resetlogs" and you have a problem, that is when you realize you made a silly mistake. I hesitate to call this a backup of the online redo logs, since I consider it part of recovery, and see #2.
  2. It is an unwinnable nearly religious war whether you are better off with the online logs on the backup sets. (My particular faith in this matter is hotbackups should never include online redo logs, cold backups only if the site procedures include step #1 and an alternate reload location naming scheme, and only do cold backups if your situation allows time for a shutdown normal.) If you do the double dip shutdown (abnormal), startup restrict, shutdown normal for cold backups, then no transactions from the online logs are needed, so if you happen to reload an entire cold backup tape to use for roll forward, all you have done is create risk that the online redo logs will be overwritten by a possibly Oracle ignorant computer operator. On the other hand, if your "cold backup" is not from a clean shutdown, if you try to load it up and start the database you will likely need transactions from the online redo logs from the non-clean shutdown. For those whose "religion" on this matter differs from mine, I suggest that you write the on line redo logs to a separate tape set and see if you ever need them in any scenario you can create on your test restore machine. (If you don't have a test restore machine, you don't take recovery seriously, anyway.) Of course this excludes the well-known fact that the cold backup of a shutdown abort is likely to need the online logs, because I advise against that particular backup anyway. In the event of a machine crash you have to evaluate whether there is time for a complete save before you restart. FIRST, get a copy of the online logs just as they are. Then think about a complete save set and/or splitting off your third plex before you procedure. This is a slippery slope argument where your particular situation and resources dictate what you should do. Usually machine crash restarts go smoothly because Bill Bridge is a very wise and careful person.
  3. If you are building internally complete hot backup sets, remember to alter system switch logfile after all tablespaces are out of backup, wait for archive to complete and include all archived redo logs from the start of the first tablespace begin backup through the redo you switched out of after the last end backup. If you're paranoid, start one earlier and switch and wait twice at the end. These are not really needed, but if the person scripting up the copy of arch for the internally complete hot backup set is fencepost challenged you still get the right answer.

(Draw a picture of a barbed wire fence and count the posts and segments of fence if you are not familiar with that term.)

May you sleep well knowing your backup that you never hope to use will work because you test it every {day|week|not much longer than that} on your test recovery system!


PS: Think about RMAN, but I've got no criticism for those who want to wear a belt and braces (suspenders) by having both RMAN backups and "hot" backups.

-----Original Message-----
[]On Behalf Of Jeremiah Wilton Sent: Saturday, October 02, 2004 6:28 AM To: Chirag DBA
Subject: Re: Question in HOT BACKUP

On Sat, 2 Oct 2004, Chirag DBA wrote:

> When I was asked which filese I will copy for hot backup, I said redo too.
> Cause I think they can help us always to recover database upto point of
> I was pulled a lot, but my only question that if not redo then how
> will you recover yr DB upto point of failure, stopped the arguements.

Askdba removed from the distribution.

If you can win the argument, you must be right! Anyway good for you for checking on this even though you convinced the interviewers you were right.

The reason that many people recommend against backing up online logs during hot/cold backup is twofold:

  1. If you back up the current online log, you have no guarantee of obtaining a clean copy since log writer might be halfway through performing a write. You should obtain the most recent log by archiving it (archive log current).
  2. If you restore a backup including the online logs (in their normal location) and the controlfile is also restored in the normal location, then the restored database can be brought up in such a way that crash recovery would take place automatically without prompts, and there could be no opportunity to apply archive logs to roll the database forward to a future point in time.

It is likely that you might want to recover past the point in time of the backup, thus the online logs would be nowhere near the most current logs for the purposes of a restore/recover exercise.

Jeremiah Wilton

Received on Sat Oct 02 2004 - 08:49:07 CDT

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