Re: What exactly is writen in the logs.

If I could add just one redundant comment for clarification - If you execute an update of 1000 rows, and then roll it back, this means that the update, as well as the re-write (the rollback) are logged to the redo logs. (assuming everything is woking normally) Is this right?

Thomas Kyte wrote:

> A copy of this was sent to USER <user_at_host.domain.com>
> (if that email address didn't require changing)
> On Wed, 10 Mar 1999 14:44:42 -0500, you wrote:
>
> >Hi, Thomas:
> >
> >I think that was the most profound clarification I've ever seen. It's a veritable
> >epiphany.
> >
> >Though a little new to this, and not having encountered an explicit description of the
> >sort you sent, this was how I'd subconsciously envisaged Oracle transaction
> >processing. I would appreciate it if you could indulge me by throwing more light on
> >this. An example scenario for an update/delete would help.
> >
> >< redo holds all after images, after images for data you are modifying as well as
> >< after images of rollback (which happen to be before images of your transaction).
> >When you say AFTER images of rollback, you mean changes to the rollback segment:
> >and only the old data is kept in the rollback segment. Thus, after images of rollback
> >are before images of the transaction in question.
> >
>
> Yes, after images of rollback (redo of the rollback) just happen to be before
> images (undo) of your transaction.
>
> >So, if you use copies of redo logs during recovery, but have lost the rollback
> >segment(s),
> >you have the results of phantom transactions that were never meant to be ultimately
> >committed.
> >
>
> you cannot use redo logs to do recovery without using the rollbacks as well. We
> will roll the rollback forward (boy does this get confusing sounding) and then
> ROLLBACK from the roll'ed forward rollback.
>
> If you zap your rollback or do hot backups and don't ever backup your rollback
> (i've seen people do that, baaaaad scene), you cannot really use your logs to
> recover. We need to recover rollback and THEN after its recovered, rollback
> from it.
>
> so, if you use copies of redo logs during recovery but have lost the rollback --
> you are "hosed". Need that hot backup of rollback to recover. Rollback and
> redo are not 100% in sync -- we need BOTH.
>
> >Thus, even if you have the AFTER image of the rollback segments, and thus the old
> >data, Oracle
> >does not know that a rollback occurred, and thus keeps these rows, resulting in data
> >inconsistency
> >of a different sort.
> >
>
> there are some magic init.ora's that might be able to do this but its only used
> to recover data for a new database you will be creating :) call support for
> this.
>
> need to have that hot backup of your rollback -- we need to recover it.
>
> >So this begs the question: Under WHAT specific conditions, can we use ONLY the redo
> >logs to reconstruct the rollback segments (as the redo logs have the information
> >needed to do this) and ignore the rollback segments altogether? (Say we set up a
> >scenario in which we deliberately blow away the
> >rollback segs, so as to test recovery.)
> >
>
> none, there are no circumstances that you can use only the redo to reconstruct
> the rollback. We need the hot backup of the rollback and we need to roll it
> forward. The redo does not have all of the information you need -- we need the
> starting rollback segment image to roll it forward.
>
> If you blow away your rollback you had better have a hot backup of them to
> recover with.
>
> >Please confirm my humble take on this, and provide a blow-by-blow scenario of an
> >update/delete if possible.
> >
>
> don't know what you would be looking for here. the udpate would get logged to
> redo (new bytes into redo), it generates rollback (old bytes into rollback),
> rollback generates log of itself. delete is similar.
>
> >Thanks so much!
> > Regards
> >
> >Thomas Kyte wrote:
> >
> >> Oracle rolls forward using REDO logs (recovering not only regular 'tables' and
> >> such but also recovering rollback segments) and then rollsback any uncommitted
> >> transactions from the rollback that was just rolled forward.
> >>
> >> >I thought REDO only captured after images, and the rollback segments
> >> >contained before images, which is why if you lost a rollback segment,
> >> >you couldn't roll back. Are you saying rollback info is placed in redo
> >> >logs as well?
> >>
> >> the after image of rollback is logged there yes.
> >>
> >> >If so, which part?
> >>
> >> all changes to rollback.
> >>
> >> >You say after images of rollback are indirectly
> >> >stored in the redo. Are you meaning that say for 1 transaction - the rollback
> >> >holds the before information, and the redo holds info about the transaction,
> >> >as well as after images?
> >>
> >> redo holds all after images, after images for data you are modifying as well as
> >> after images of rollback (which happen to be before images of your transaction).
> >>
> >> rollback holds before images.
>
> >>
> >> >In that case, no before image or anything from the
> >> >rollback segments themselves are really sent to the redo logs -- right?
> >> >
> >>
> >> right, only after images of rollback (which are before images of your
> >> transaction) are sent to redo.
> >>
> >> >- Dc.
> >>
>
>
> Thomas Kyte
> tkyte_at_us.oracle.com
> Oracle Service Industries
> Reston, VA USA
>
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Received on Tue Mar 16 1999 - 09:17:47 CST