Re: hot backups question for experienced DBA's

You don't recover the redo logs (just archived ones). The redo logs tell Oracle recover command important information that cannot be recovered (like current SCN - system change number). Sounds like you are aware, but obviously the online redo logs are the most important files in Oracle - that's why they are the only ones that can be mirrored (multiplexed) .

"Aznable" <termy_at_blunet.it> wrote in message news:3AE27A3D.ED28EDE1_at_blunet.it...
> > ...
 

> > As a test, I run the backup script and then wait a few hours so that a
> > couple more archived redo logs are created. I then run a script that
> > wipes clean the entire database (simulating a catastrophic failure of
> > all disks). All datafiles, control files, redo logs and archived redo
> > logs are wiped out. I then copy back all the files from the spare disk
> > and attempt a recovery. Invariably, Oracle ends up requesting one of
> > the archived redo logs that was created AFTER the backup completed.
>
> If you enable ARCHIVELOG mode on database you can recover
> the DB to the Point of Failure, so after the DB has been backued up.
> e.g. U take a backup the midnight of 1st of march and U have a disaster
> at 15.00 of the same day. If U have archived redo logs and online redo
> logs U can recover the database until the distaster happened (all commited
> transaction are recovered).
> If you want to recover until the bckup time you have to do an incomplete
> recovery (e.g. UNTIL TIME, UNTIL CHANGE), but always remember that
> during the backup process (i guess U put the tablespace in backup mode)
> more redo are generated and U should force the archiving of current log
> and bakup it also.
>
> > One thing I find interesting is that the book makes no mention of the
> > need to backup the online redo logs, just the archived ones. Why are
> > the redo logs not necessary? Since they are the ones to which Oracle
> > is writing and the archived logs are only created when an old redo log
> > is about to be overwritten, it would seem logical that the current redo
> > logs are far more important for recent recoveries than the archived
> > logs.
>
> Because when U commit a transaction Oracle doesnt write on datafiles
> where tables reside but it writes in the online redologs only (the writing
> on
> datafiles is deferred at checkpoint or log switches events).
> In event of a crash (even minor) the online redologs are used re-apply the
> trasactions
> not written to datafiles.
>
> > Right? Wrong? Do I need to backup the current online redo logs as
> > well? If so, why does the DBA handbook not mention this fact?
>
> In an hot backup U dont have to backup online redologs. But U
> can multiplex the members for each group spreading the across different
> disk so will be unlikely U will miss all of them in case of failure.
>
> BTW You should read something about the Oracle's architecture and
> how the things works. (e.g. why U need online redo, when DBW writes
> modified blocks to datafiles and so on..)
>
>
Received on Sun Apr 29 2001 - 01:01:58 CDT