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Re: what happens if you lose a rollback segment?

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Sun, 19 Oct 2003 06:27:57 +1000
Message-Id: <3f91a2db$0$21654$afc38c87@news.optusnet.com.au>


Ryan wrote:

> yes Im AWARE of that. However, there are different types of failures. Lets
> say all your datafiles are fine, but you lose a Rollback segment during
> the day at say 1PM. You have alot of uncommitted transactions.

That means there's a lot of data in that rollback segment which (a) tells us the transactions are uncommitted. And (b) the only way we know what to roll those transactions back to is by looking at the data that rollback segment has within it.

But you've just lost it. OK...

> Therefore you are forced to restore from last nights backups right? The
> rollback tablespace is from LAST NIGHT.

Correct. At which point, of course, the rollback segment in question is full of old, stale data, not the important stuff we need for points (a) and (b) above. Fair enough...

>So your rolling forward for 12
> hours,

...And as you roll forward, you *repeat* all the transactions you did for 12 hours. And as you do that, you *re-generate* all the rollback those transactions produced. And so as you approach the 11 hour 59 minute mark, your rollback segment is nowhere near as stale and old as it was when you first restored the datafile. By replaying redo, you refresh its contents, just as you did when you first performed those transactions.

At the 12th hour, the rollback segment looks identical to the one you originally lost. It contains all the data present originally and needed for points (a) and (b) above.

>all the rollback information in your nightly backed up rollback
> tablespace is outdated by then. So why do we take backups of rollback
> tablespaces?

What you restore is one thing. Outdated, old, stale. What you get as you repeat transactions during recovery is fresh, new and up-to-date. That goes for rollback segments just as it does for any other segment (table, index whatever).

> or do you take backups of rollback tablespaces more often than datafiles
> that way if you lose one you dont have to restore from such an old copy?

Nope. The backup frequency should be exactly the same for RBS as it is for DATA, INDX or SYSTEM.

The only thing I think you've missed, or appear to have a block on, is that by replaying redo, you make old, restored rollback segments as up to date as an old, restored EMP would become by doing the same thing.

Regards
HJR

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Received on Sat Oct 18 2003 - 15:27:57 CDT

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