Re: Online backup: Backup online redologs?

"Howard J. Rogers" wrote:

> "Vikas Agnihotri" <onlyforposting_at_yahoo.com> wrote in message
> news:ata1cts3d70qan0bkbo9oa94i2eccsd0bd_at_4ax.com...
> > Kevin Loney's Oracle 8 DBA handbook mentions the following 3-step
> > process for a online/hot backup:
> >
> > 1. Backup each tablespace by putting it in backup mode
> > 2. Backup the archived redologs (and optionally delete them to save
> > space)
> > 3. Backup the controlfile using ALTER DATABASE BACKUP CONTROLFILE
> >
> > Why is there no mention of the online redologs?
>
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAARRRRRRRRRRRRRRRRRRRRRRRGGGGGGGGGGGGGGGGGGGHHHH
> HHHHHHHHHHHHHHHHHHHHHH!
> I don't beleieve this.
> This was discussed at ENORMOUS length just last week. Don't you ever scan
> other posts before sending off your own??
>
> Really.
>
> OK. <calm>.
>
> The basic rule in Oracle is this: you can't copy anything hot unless you've
> a mechanism for making it consistent subsequently, because all hot copies
> will be internally one big mess. Different bits copied at different times.
>
> Datafiles copied hot will be inconsistent. However, if you apply redo to
> datafiles, they become consistent, and can then proceed to be rolled forward
> to whatever time (read: SCN) you choose.
>
> Controlfiles copied hot will be inconistent. And you can't apply redo to
> controlfiles. So they can't be made consistent... but fortunately, the
> 'alter database backup controlfile to 'c:\blah\control01.bkp'" command makes
> Oracle do the copying, and when Oracle is in charge it can use tricks and
> techniques to guarantee the output will be read consistent. (Woe betide
> you, however, if you *ever* do a simple O/S copy of a hot controlfile,
> because it will be unuseable).
>
> Redo Logs copied hot will be inconsistent. And you can't apply redo to redo
> logs. So they can't be made consistent.... and UNfortunately there is no
> 'alter database' command that will generate consistent copies. So any hot
> copies of redo logs will be inconsistent, will always be so, and will thus
> be totally unuseable.
>

My hot backup, working whit 8.0.6, makes, at the end, a hot copy of the online redo logs, including the current. And, at recover time, I didn't see any problem so far.
I know, if it's possible, I'll preserve the redo log files on disk (after a failure).
What are the options for a total failure of a database with a hot backup and some archived redos?
I tried to recreate the redo logs in mount mode, but 'he' is always asking me about the lost redos.

>
> And then there's this: think about it. If you are in archivelog mode, you
> *do* have a consistent copy of the redo logs in the archive log (which is
> 'cold' the second it is written). So by backing up the archivelogs, you are
> in effect backing up consistent copies of the online redo logs. The only
> exception is the CURRENT redo log, which is hot, hasn't been archived, and
> cannot be O/S copied. This is the potential achilles' heel of the Oracle
> database. Loss of the current log will result in data loss -but that's why
> redo log mirroring was invented, to make the potential total loss of the
> current log extremely unlikely.

...but possible...

>
>
> >During BEGIN BACKUP,
> > arent the changed blocks written to the online redologs (in addition
> > to the datafiles themselves)? So, if my entire backup is over before
> > the online logs cycle, my online backup will be missing the changes
> > generated during the backup since they are present only in the online
> > redologs, right? What am I missing?
> >
>
> You are missing the usual recommendation from Oracle, which is to issue an
> 'alter system switch log file' command at the end of each tablespace's spell
> in hot backup mode.
>

I'm making a switch log at the begin and another switch at the end of all tablespaces. Am I right?
Or are you talking about backing up only one tablespace?

>
> > And if the online redologs should indeed be backed up, shouldnt there
> > be a BEGIN BACKUP for them as well since they are in a state of flux?
> >
>
> See above. It makes no sense to backup the online redo logs. They've
> already all been backed up to the archives, with the exception of the
> current log. If you are that concerned, then a switch log file will cause
> that to be archived, and by backing up that last archive, you've got the
> lot.
>
> Regards
> HJR
>
> > Thanks...

Thanks in advance by your opinion HJR.

José Nicolau Received on Sat Mar 31 2001 - 08:04:08 CST