Re: Frequent log switches during sqlldr batch jobs

Frank van Bortel <fvanbortel_at_netscape.net> wrote:
> xhoster_at_gmail.com wrote:
> > "Thorsten Jens" <thojens_at_gmx.de> wrote:
> >
> >>Frank van Bortel wrote:
> >>
> >>>Take about 5 switches per hour, then. Gives you 12 minutes
> >>>worth of transactions to loose when the current redolog file
> >>>crashes.
> >>
> >>If I sized the redo logs to have ~5 switches per hour during high load
> >>times, then I'd have almost no switches at all during the day -- which
> >>would mean a lot of lost work in the worst case.
> >
> >
> > I've seen this argument many times, but I don't understand it. Isn't
> > the worst case the one where both the online redo log and recently
> > archived redo logs are lost? In most systems I see, the online redo
> > logs are protected by mirroring and multiplexing to the same extent as
> > the freshly archived rego logs are (if not more). Is this not usually
> > the case?
> >
> > Xho (not a backup/recovery DBA)
> >
> Both? What do you mean both online redo logs?

I meant both of the two types of redo logs, "online redo logs" as one group and "archived redo logs" as the other group.

> And of course, the more you lose, the worse it is.
> Guess the worse case would be a fire, destroying your
> hardware: all online and not backed up archived redo
> logs get lost (and hopefully your tapes are off site!!).
> You loose all transactions from the point where your
> last backups of the archived redo log files is.

OK, so in a slightly less bad case, let's say that some calamity causes some disk files to be lost. Is there any reason to think the online redo logs are more likely to be the ones that are lost than the archived redo logs?

There seems to be an assumption that transactions are "safer" once the log file in which are committed is archived. I can see how that would be the case in the long term, when arhived logs are backup up to tape (but in the long term, all logs become archived, so that kind of cancels except at the moment before the copy-to-tape is initiated.). I can see how that would be the case if online logs are duplexed and mirrored while archived logs are tetraplexed and mirrored (or something like that). But if arhived logs are treated with the same redundancy as online logs, is it true that transactions are safer once the corresponding log has been archived?

Cheers,

Xho

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