Re: Question about Archivemode

"Ryan" <rgaffuri_at_cox.net> wrote in message news:H1nZ8.92774$%%2.3974480_at_news2.east.cox.net...
>
> "Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message
> news:ah4rcd$en2$1_at_lust.ihug.co.nz...
> >
> > "Howard J. Rogers" <howardjr2000_at_yahoo.com.au> wrote in message
> > news:ah4r3k$ecu$1_at_lust.ihug.co.nz...
> > >
> > > "Ryan" <rgaffuri_at_cox.net> wrote in message
> > > news:R4lZ8.92701$%%2.3951689_at_news2.east.cox.net...
> > > > 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.
> >
> > BONG! I meant to say 12.00 noon.... you've 2.5 hours of redo available,
> and
> > he swipes it three hours after the database is opened... *then* you're
> > doomed. 11.00am would be OK, because (despite my pathetic maths) that's
> only
> > 2 hours after being opened for business, so the required redo should
still
> > be available.
> >
> > Sorry. (Walks away muttering to himself.... "9+3=12, not 11", "9+3=12,
not
> > 11")
> >
> > Regards
> > HJR
> >
> > >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.
> > >
> > > Regards
> > > HJR
> > >
> > >
> > > > For those of you who saw an earlier post of mine... no this did not
> > > happen.
> > > > :)
> > > >
> > > >
> > >
> > >
> >
> >
>
> thanks howard... my articulation is atrocious.. I meant use the redo logs
to
> recovery. Sorry, but thanks for the explanation... didnt recall it in all
> that much detail.
>
> Another question. We are using a tape backup. I do not know how these
things
> work. If the database was to get destroyed for some reason, but the hard
> drive is good.. will I need to reformat the hard drive and reinstall all
> oracle products to get my backup with a tape or can I just grab the
> datafile, redo log files, control files... etc..

It entirely depends on what you mean by "the database is destroyed". If you mean someone has wiped all datafiles, online redo logs, controlfiles and on-disk archives, but otherwise the hard disk is functioning fine, then there's no need to wipe it and re-install Oracle. You just restore the datafiles, re-create the controlfile and open with a resetlogs. If you've been taking cold backups because you are not in archivelog mode, then you may well have the online logs included in your backups... so you can restore them, too, and re-create the controlfile, and open the database without a resetlogs.

I suppose, though, the real question is how the stuff got onto your backup tapes in the first place. If you've used RMAN to put them there, then things get very interesting indeed. Assuming you are using RMAN with a recovery catalog, that catalog needs to be recovered itself before you can use RMAN to read the tapes containing your main database files. But if all you've been doing is using something like backup exec or arcserve to backup the files onto tape, and assuming those programs are still functioning, then you just restore the database files like I said.

Short of user stupidity, it is practically impossible to wreck an Oracle database to such an extent that you would lose absolutely everything. Even in noarchivelog mode, it is always possible to get something working again. You might lose some data, depending on how old your backup was, but you'd have to be criminally incompetent to mangle things so severely that I lose the lot, utterly, and completely, and without possibility of some sort of recovery.

Regards
HJR
>
>
Received on Wed Jul 17 2002 - 19:01:38 CDT