Re: what happens if you lose a rollback segment?

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Sun, 19 Oct 2003 06:18:34 +1000
Message-Id: <3f91a0a7$0$21650$afc38c87@news.optusnet.com.au>

Ryan wrote:

>>
>> --
>> Sybrand Bakker, Senior Oracle DBA

>
> I understand, but since you need to restore from a backup datafile if you
> lose your rollback segment. What is the point of backing up your rollback
> tablespace? You have to restore from your 1AM datafile, then roll forward.
> The rollback segment you would have backed up at 1AM wouldnt be any use if
> you are rolling forward until 1PM?
>
> Im sure Im missing something here?

It's a good question, actually.

First, there's the answer that says any recovery consists of being able to roll everything forward via the redo logs. Then you get to the end of the redo, and start rolling everything back that wasn't committed. You need rollback segments for both of those phases. During the roll forward phase, we replay your original transactions exactly. So if you generated some rollback when you first did the transaction, we have to be able to generate it again during recovery. And during the rollback phase of recovery, we obviously use rollback generated during the roll forward phase.

Hopefully, that's enough of an explanation to persuade you that rollback segments *in principle* are vital for all recoveries.

But that begs the question: why can't you just create new rollback segments to perform recovery? Why do you have to have the original ones??

To which the answer is: redo consists of change vectors. That is "put this value, in that block of object X, on data file Y". And that goes for rollback data, too. Were you to create a new rollback segment, you'd be talking about object A, not X. And the redo change vector couldn't be applied. And if we couldn't therefore re-generate your rollback data properly, we couldn't perform recovery properly.

Regards
HJR

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Received on Sat Oct 18 2003 - 15:18:34 CDT