Re: Hot backups and rollback segments.

Thanks for your reply. I incorrectly assumed that when a tablespace is put in to backup mode all updates were only written to the redo log and not the datafiles. Hence you could use an OS backup utility safely. I now understand that writes are being made to the datafiles at the time of the backup, and that possible corrupted blocks are not a problem because these blocks are restored from the redo logs.

My premiss, I believe, is still correct that Oracle could use the rolling forward process to generate rollback information for transactions that need to be rolled back. However, It would still need information from the roll back segments for those transactions that started before the earliest datafile SCN as the required information would not be in the redo logs at the point it started rolling forward.

Thomas Kyte wrote:

> A copy of this was sent to Mike Burden <mburden_at_uk.att.com>
> (if that email address didn't require changing)
> On Mon, 18 Jan 1999 11:58:23 +0000, you wrote:
>
> >I've recently been playing about with Oracle in Archive Log Mode and Hot
> >backups but one or two things are confusing me.
> >
> >I understand the need for all the data files to be in line before Oracle
> >allows access to the data but I'm a bit confused as to why the rollback
> >segments need backing up.
> >
> >Q1. Do you need to hot backup the rollback segments??? You can 'begin
> >backup' on the tablespace but what on earth is this doing. Surly it
> >doesn't write the information for the rollback segments to the redo
> >logs.???? How can the system keep on running if the roll back segments
> >are not writeable whilst a backup is taken.
>
> Yes, backup rollback, yes, yes, yes.
>
> it surely does write the REDO information for rollback to the logs. ROLLBACK is
> logged just as DATA or INDEX segements are.
>
> Upon recovery (say from an instance crash, power failure), REDO is applied from
> the logs to roll forward -- including rolling forward ROLLBACK segments. Then,
> rollback is used to roll data back (uncommitted transactions).
>
> >Q2. Why does the RBS tablespace need to be synchronized with the other
> >dbf files. Surly once you've rolled forward to a point in time (or
> >whatever) then the contents of this table space are irrelevant as all
> >truncations are now complete. Only the structure of the rollback
> >segments is important for a recovery, not their contents?
> >
>
> because rollback rolls back. During recovery, you will apply REDO rolling stuff
> forward. At some point you'll 'finish' rolling forward -- now you need to
> rollback uncommitted transactions at that point in time, enter rollback.
>
> You need the rollback, its extremely important.
>
> >Q3. Is it possible to tell Oracle that the contents of a dbf file or
> >tablespace are not required? Say for instance you've backed up all
> >required datafiles but forgotten to back up the the users table space.
> >You knows there's nothing on it you require but somehow you need to
> >indicate this to Oracle. This would be useful for the RBS tablespace.
> >
>
> no it wouldn't (be useful for the rbs tablespace) because you really DO need it.
>
>
> Thomas Kyte
> tkyte_at_us.oracle.com
> Oracle Service Industries
> Reston, VA USA
>
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Received on Tue Jan 19 1999 - 02:49:17 CST