Re: odd number of rollback segments

Nick Keighley wrote:
> Nick Keighley wrote:
> > Hi,
> >
> > I gather questions about Rollback Segments are quite common...
> >
> > I'm running Oracle 8, and no I'm not going to be able to upgrade soon.
> >
> > My system seems to be using a lot of disc space with ARC files. It's
> > generating
> > about 10 ARC files /min. I understand this is Bad and may indicates
> > too few
> > (or too small) rollback segments. Today I got an ORA-01555 for slowish
> > transaction.
> > Again this is probably related to RBSs.
> >
> > Only two public rbs appear to be specifically created by the startup
> > SQL.
> > So presumably Oracle created the rest. There are 7 (RBS0-RBS6).
> >
> > I've dug back though various configuration parameters
> >
> > processes=50 (sounds big for my environment)
> > sessions=60
> > transactions=66
> > transactions_per_rollback_buffer=5 (TPR)
> >
> > hence Oracle should allocate transactions/TPR=13.2 public RBS
> > and not a measly 7. Why?
> >
> > What do I have to do to change the number of processes? Update the
> > init.ora
> > and restart the database?
> >
> > My rollback segments are all 2M except for number 6 which is 4M. Why?
>
> eek! I have now found the file that is really building the database. It
> creates
> 7 RBS...
>
>
> --
> Nick Keighley

Nick, arc files are from the archived redo logs and have no direct relationship with rollback segments.

If you are genenerating multiple archived redo logs files per minute it means that your online redo log files are allocated too small. You can find instructions in the DBA Administration manual on how to add and drop redo logs files while the system is in use. Basically add a new larger one, then drop any non-currently in use archived log.

It is very possible that 7 RBS segments are all that your database needs. How many concurrent transactions, not user sessions, do you see in v$transaction. Is your database an OLTP or DSS/OLAP type system? How large (amount of data) is the average transaction on your system. Often a lesser number of larger rollback segments worked better than a larger number of smaller segments.

HTH -- Mark D Powell -- Received on Thu Aug 31 2006 - 10:51:37 CDT