Re: Why Backup Temp and Rollback segs?

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.

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 Received on Mon Oct 19 1998 - 10:24:45 CDT