If Waits/s are high for a given class, you may consider some tuning: For the undo headers/blocks, adding more rollback segments can help. With data blocks, increasing the size of the database buffer cache can reduce these waits. Segment header waits generally point to the need to add freelists to the affected table. Freelist block waits indicate that the affected segment needs a higher number of freelists - for the Oracle Parallel Server, make sure each instance has its own freelist groups.