Re: Question about Undo segments

No. If you're in auto mode, and use the set transaction command, you'll error. However, there's a parameter, undo_suppress_errors, which -if set to TRUE- prevents all the code you have containing such a command from bombing out disastrously. But the command is nevertheless ignored.

And quite right too! The point with auto is that Oracle is managing the whole show for you. Your job is simply to provide sufficient disk space, spread across sufficient spindles. The rest just happens!

Now you will probably say that this is dreadful: that all the I/O of the bulk job will be hammering the I/O of the concurrent little jobs, whereas in the old days, using that set transaction command, you could ensure they were separated from each other. True enough: that's why there's a premium these days in auto mode of having your total undo I/O spread across as many devices as possible. You can thus achieve the same result, of non-I/O-contending undo generation, but themechanism is rather different.

Regards
HJR "Peter Sylvester" <peters_at_mitre.org> wrote in message news:3CCD881D.3F6F81D0_at_mitre.org...
> As a followup on this auto vs manual redo/undo...
>
> If you are running 9i in auto redo mode, can a session specify that it
> wants to use an old fashioned manual RBS (via SET TRANSACTION) for a
> redo intensive load job?
>
> tx
> Peter
>
> "Howard J. Rogers" wrote:
> >
> > "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 Mon Apr 29 2002 - 13:32:13 CDT