Re: Hot Backup - Need for Rollback Segs

> LNudelm_at_unotes1.mnet.uswest.com (Larry Nudelman) writes:
> I am reviewing several Hot Backup UNIX Scripts that back up ALL! the
> DBFs and Control files. They obtain the list of dbf's from
> v$datafile, which includes the rollback dbf's. Based on all the
> restore notes, only data files, redo logs, archive redo logs, and
> control file are needed for either a partial or full restore. So why
> back up the Rollback dbf. I can't see restoring a RB tablespace from
> the hot back up if the OS file gets hit.
>
> The rbs tablespace is about one-fourth the size of the entire database
> so if I can exclude it from the hot backup I could save a lot of time
> and space. Any comments on if I can / should exclude them.
>
> Thanks,
> Larry Nudelman, DBA
> U S WEST MIS
>
>
>>>>

When Oracle writes to the rollback tablespace, or any tablespace for that matter, it writes to the redo logs, which become archive logs.

I may be pointing out the obvious, but when you must restore your database, Oracle will attempt to write to all tablespaces, even the rollback tablespace.

It's possible (albeit tedious) to tell Oracle to only restore certain tablespaces, but you're taking a BIG risk by skipping any tablespace. Unfortunately, by the time you find out you needed the tablespace, it's too late.

Also, just because the tablespace is where you store your rollback segments, it's possible for other objects to accidentally end up there. This would probably not happen if you are careful, but what happens when you go on vacation?

Another issue is that it's a pain in the neck to start up a database that is missing a tablespace. Consider this: If your disk crashes, here are your steps to recover (simplified):

1. fix the disk
2. restore all .dbf files, control files, etc. from the last warm backup
3. start up the database
4. apply archive logs

If you are missing your rollback dbf files, step 3 will crash. There is a sequence you can go through to get around this, but it's not something you want to plan to do.

Another idea is to keep a cold backup of the rollback dbf files, and not back them up each night. This sounds good, but Oracle keeps information in the header of each file (system change number I think). When you try to do step 3, Oracle will think the file is corrupt, and will try to apply redo entries to the file. Again, there is a way around this, but you don't want to plan on it.

It's just my opinion, but the savings in time and money is miniscule compared to the peace of mind of having a good backup. I usually even back up the database using more than one method. (warm backup & full export to disk).

Will Kooiman Received on Fri Aug 11 1995 - 00:00:00 CEST