Re: Why Backup Temp and Rollback segs?

Jeremiah Wilton wrote:

> On Mon, 19 Oct 1998, Graham Thornton wrote:
>
> > Reading through the Oracle DBA handbook, I noticed that the section
> on
> > on-line backups includes examples of backing up the TEMPORARY and
> > ROLLBACK datafiles.
> >
> > Why would you want to do this? - If you need to recover from a crash
> you
> > can only recover committed transactions so why would you need the
> > TEMPORARY and ROLLBACK datafiles? - Wouldn't it be easier to just
> > recreate them?
>
> The important thing to realize is that during normal operation of the
> database, the data blocks (in the block buffer cache and the
> datafiles)
> are changed by transactions, even before they are committed. The
> before
> images of uncommitted transactions are placed in the rollback
> segments. So
> if the database crashes, you still will need to roll back those
> uncommitted transactions, moving the data back from the rollback
> segments
> to the data blocks.

> A common misconception (what, another??) is that the data blocks are
> not
> actually changed until a commit is issued.. This is incorrect. It
> appears
> this way from within other sessions, because those sessions know, by
> looking at the ITL (interested transaction list) entries in the block
> headers, to read the before-images of the changed blocks out of the
> rollback segments.
>

OK, slow down, slow down... I'm confused now. What happens in the event of a "snapshot too old" error? If the transaction has changed the data, how do you rollback to your original state in this case?

> A TEMP tablespace could be dropped from the database and recreated
> with no
> loss of data in the event of a recovery. This just adds one more step
> to
> your recovery procedure. If your TEMPORARY tablespaces are large,
> this
> may be a good option.
>
> --
> Jeremiah Wilton http://www.wolfenet.com/~jeremiah

mkb Received on Mon Oct 19 1998 - 12:05:57 CDT