odd number of rollback segments

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?

-- 
Nick Keighley