Originally posted by Sureshy
Hi Friends,

I have 25 RBS
looking in DBA_Segments, the are all different sizes and all have a different high water mark, they are not set to an optimal size yet and all have the same storage parameters(Initial, next, min, max)

When the subject of increasing rollback segments is discussed, what does this actually mean.

I always thought that rollback segments were dynamic and would grow and shrink dependant on their storage parameters, only limited by the size of the tablespace they live in or their max extents.
Sort of yes, sort of no. Typically, RBS grow with larger DML statements. Unless you have an OPTIMAL setting, they will not shrink. Extends and shrinks should be limited in rollback segments to optimize performance.

Remember, RBS are also used for SELECT statements so you get a consistent view of the data and will NOT expand the RBS. If this is the case, you will receive an ORA-1555 error and you should adjust the OPTIMAL parameter of your RBS.


Does increasing the size of RBS mean giving them a larger optimal size, or larger amount of min extents ?
In my mind, I think that means that the OPTIMAL size of the rollback segment needs to be adjusted.


Also..

What is the advantage of issuing :-
Set transaction use rollback segment XXXXX. -
I always thought that this would allow a session to maybe use a large RBS for a large transaction, but as all RBS's are dynamic an would grow/shrink depending upon the transaction and optimal size, would it not be equally as proficient to use any rollback segment /
IMHO, If you have properly sized your rollback segments, there is no advantage. RBS are assigned on a round-robin basis and your transaction should be able to use any available RBS.



Someone Please enlighten me.

The reason I am asking is because I have been asked to increase RBS size by a pseudo DBA, who is my senior and I need to be armed with a more clearer understading of them before I tackle this subject.

Many Thanks

Suresh
I'm sure he/she is experiencing a problem. Ask them why they think the RBS need to be increased.