ORA-01555: snapshot too old: rollback segment number string with name "string" too small 

Cause: Rollback records needed by a reader for consistent read 
are overwritten by other writers. 

Action: If in Automatic Undo Management mode, increase the setting of UNDO_RETENTION. Otherwise, use larger rollback segments. 

 When using Oracle Export utility, setting CONSISTENT=Y 
creates a read-consistent export. That is the export is 
consistent to a single point in time. This requires the 
retention of rollback segments if transactions modify the data 
while the export is in progress. Set CONSISTENT=N if you dont 
need a read-consistent export of if a large volume of the data 
will be modified during the export. Also, schedule exports 
during off-peak hours.