Re: Rollback Segments Question

DA Morgan <damorgan_at_exxesolutions.com> writes:

> Comments interspersed
>
> Tim X wrote:
>
> > I have a couple of questions relating to rollback segments I am hoping
> > to get some clarification on from this group. The database is 8.1.7 on
> > both True64 and Linux boxes.
> >
> > 1. According to the 8i documentation, you need to create an additional
> > rollback segment when creating a new database. I've also seen this
> > mentioned in a couple of books with the rationale that this is
> > needed in order to create additional tablespaces and rollback
> > segments in the db creation script. My problem with this is that I
> > have seen databases created and have created databases where I did
> > not need to create an additional rollback segment in the system
> > tablespace in order to create tablespaces or rollback segments. So,
> > what is the story here. Is this something left over from earlier
> > Oracle versions and is not a requirement in newer versions (and
> > just has not been removed from the documentation) or is there
> > something subtle I'm not aware of?
>
> This is likely a reference to locally managed tablespaces (LMT). When creating
> rollback segments in an LMT there must be one additional dictionary managed
> rollback segment on-line in order to create the LMT tablespaces and the initial
> rollback segment in an LMT tablespace. As soon as the first LMT rollback segment
> is created the additional rollback segment can be taken off-line and dropped.
>
> >
> > 2. According to the Oracle 8i documentation, if you do not include
> > your rollback segments in the rollback_segment parameter in the
> > init.ora file, your rollback segments will not be brought back
> > on-line when you bring your database up and will need to explicitly
> > be brought on-line with a alter statement. THis is in addition to
> > the rollback_segments other role relating to public/private rbs and
> > parallel servers. I have done some experiments with 8i and this
> > does not seem to be the case. If I don't include the RBS in the
> > parameter file, they still come up on-line when the db is
> > started. Again, is this just old documentation left from an earlier
> > Oracle version, a documentation error or am I missing something?
> >
>
> Rollback segments in the init.ora force additional segments on-line, in addition
> to those Oracle might choose to bring on-line.
>
> >
> > 3. Can you use locally managed tablespaces for rollback segments?
> > According to the Oracle 8i documentation, you cannot use the local
> > automatic option for LMT if you want to use the tablespace for
> > rollback segments. to me, this implies you could do it if you use
> > the "local uniform" option instead. However, when I attempt to do
> > this, the rollback segment creation fails with the error
> >
> > ERROR at line 1:
> > ORA-01552: cannot use system rollback segment for non-system tablespace 'RBS'
>
> Yes. See my comments above.
>
> >
> > where RBS is my LMT and RBS01 is the name I am giving to the rollback
> > segment. If I drop the tablespace and re-create it as a DMT, then
> > everything is fine.
> >
> > Any info or pointers greatly appreciated.
> >

danieal,

Thanks for the feedback. This clarifies things for me. So, the second (temporary) rollback segment in the system tablspace is required if you want to create rollback segments on locally managed tablespaces. If you are not using LMT, then I gather you don't need this second RBS?

What really had me confused here is that older Oracle documentation (pre 8.1.7) also states you need this requirement, but you did not have LMT back then (e.g. 7.3.4) - the DBAs where I work said it wasn't needed and they never use a temporary second rollback segment in their creation scripts.

THis is what I like about this group. I find often Oracle docs state you need to do something, but don't give any reasoning as to why - just do it and don't ask. I find things a lot easier to rmember if I can understand why something is needed.

Thanks,

Tim

-- 
Tim Cross
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