mooks,

To get the clear picture of the rollback segments contention, you have to look at the ratio between the total number of request for data over the same period of time and the statistics in v$waitstat, not just the numbers in v$waitstat.

Get the statistics with:

select class, count
from v$waitstat
where class like '%undo%;

Then get the number of consistent gets:

select value
from v$sysstat
where name = 'consistent gets';

The ratio of waits for any class should be less than 1% of the total number of requets. The solution is to add more rollback segments.

For rollback segments sizing you can also check:
http://www.dbaclick.com/cgi-bin/ib3/...=ST;f=20;t=279

Hope that helps,



--clio_usa
Senior Oracle DBA