Re: Question about Undo segments

"Md Irfan" <irfan_pk_at_hotmail.com> wrote in message news:c42168e7.0204280827.47e90793_at_posting.google.com...
> "Howard J. Rogers" <dba_at_hjrdba.com> wrote in message
news:<aafd03$9ki$1_at_lust.ihug.co.nz>...
> > Undo segments are just rollback segments with a new name. Therefore,
what
> > was true for rollback segments remains true for undo segments.
>
> But I have seen there are both RBS and Undo Tablespaces in 9i. Which one
is
> used for rollback.
> Is data written in both the tablespaces?
>

No. "Undo segment" is just the name we give to rollback segments when 9i uses the automatic method of managing them. If you use the manual method of managing them, they are still called rollback segments. Obviously you can't simultaneously be using both the automatic and the manual method of managing them, so it is impossible to have both undo segments and rollback segments being used simultaneously. It's one or the other.

Set undo_management=MANUAL, you have rollback segments. Set undo_management=auto, you have undo segments. Which ever one you are NOT set to, if it exists at all, remains dormant and unused. If you've created rollback segments, and switch to AUTO, those rollback segments are not written to at all. If you have an undo tablespace and you switch to MANUAL, the undo tablespace is not used at all -it's just dead disk space.

Regards
HJR
> >
> > Therefore, it's your server process that modifies undo buffers. They get
> > flushed to disk as dirty buffers whenever DBWn takes it into its head to
do
> > any osrt of disk writing. The redo for them is the same as it is for any
> > modification to any buffer: the before and after image.
> >
> > In short, there's nothing magic about them -and if you know how rollback
> > blocks are dealt with, that's how undo blocks are managed, too.
> >
> > Regards
> > HJR
> >
> >
> > "Jargular" <zclai_at_yahoo.com> wrote in message
> > news:8ae4ae9e.0204270719.22c0f769_at_posting.google.com...
> > > Hi,
> > >
> > > Does anybody know what is the process that writes to the Undo
> > > segments? under what condition the Undo buffers are flushed into Undo
> > > segments? how are the changes in Undo buffers recorded in the online
> > > Redologs? I seached Oracle9i documentation, it seems that they are not
> > > detailed in the docs. Anyone can help?
> > >
> > > really appreciated.
> > >
> > > Jargular
> > > zclai_at_yahoo.com
Received on Sun Apr 28 2002 - 14:26:35 CDT