Re: what happens if you lose a rollback segment?

Ryan wrote:

>> --
>
> I get it now. However, its common to stagger your hot backups. Say do a
> few data files a night. Lets say you have this instance.
>
> You have 3 data files and 1 rollback tablespace. You backup 1 data
> file/night.
>
> Do you need to backup your rollback tablespace every night?

No. I hate to keep saying this, but a rollback segment is just a data store like a table is. If you had to backup your rollback segments every night, you'd have to back all your tables up every night. And then if you were doing tables every night, you'd have to do rollback segments every 12 hours. And then...

...and then you'd end up disappearing up your own bottom.

If your rollback segments get backed up every Sunday, and everything else gets backed up each night, when you lose your rollback segments, you restore the relevant datafile from last Sunday. You then apply redo to it, and hence bring it up to date.

Rollback tablespaces are no different to any other tablespace. Backup when appropriate, and recovery means restore from the last backup (whenever it was taken) and make it current by applying redo.

> Also, lets say
> you are doing a hotbackup of datafile A and you are doing a hotbackup of
> your rollback tablespace. I would assume you need to keep your datafile
> and your rollback tablespace in 'backup' mode for the same period of time.

No, and I'm getting a bit lost with your assumptions. Think of a rollback segment as though it were called EMP and treat it accordingly. They are really no different from any other segment.

> If you backup your datafile first, then backup your rollback tablespace
> they could be at two different SCNs.

If you back up SYSTEM, then backup DATA, they WILL be at two different SCNs. Who cares?? What happens when you apply redo to ANY tablespace? It rolls forward in time, its SCN increments, and it therefore achieves eventual synchronisation.

The process of database recovery is the process of making different SCNs on different datafiles the same. Works regardless of what's in the data files.

>This means the rollback needed to
> restore the datafile may not exist in the rollback tablespace at the time
> of the backup.

Rollback is NOT needed to restore a datafile. Redo is. When you replay redo to replay your 'update EMP' transactions, the rollback for that update is re-generated. Therefore, when we discover at the end of the redo log that your update EMP transaction wasn't committed, we can now roll it back because the rollback segment is full of the necessary rollback.

You're making a mountain out of a very small molehill on this one.

Just one last time: rollback segments need no more backing up than SYSTEM. They are backed up in exactly the same way as SYSTEM or DATA or INDX. They are recovered in exactly the same way. They are needed for recovery, but recovery also re-constructs them just as it reconstructs transactions in EMP or DEPT.

> am I correct here?

Not really, no. Just one last time for this one too: think of a rollback segment as a table that's there for Oracle's internal housekeeping, but a table nonetheless. And treat it as you would any other table.

Regards
HJR

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