During masking operations the masking job fails and the dmsuite.log file shows errors like:
java.sql.SQLException: ORA-01555: snapshot too old: rollback segment
number 20 with name "_SYSSMU20_3360423369$" too small
This is caused when the UNDO space is too small for the size of the commits that are being performed. For masking operations, it is best practice that the UNDO space be as big as the largest table that is being masked.
Often "masked databases" are virtual databases (VDBs) that are either manually masked or are created as a masked VDB. In this case, the UNDO space size is inherited from the parent like any other configuration option. If you change the UNDO space on the VDB and then refresh it or rewind it, the operation that has just been performed will undo any changes made to the VDB. If the database that you are trying to mask is a VDB and does not have enough UNDO space, you have two options for how best to proceed:
Increase UNDO space in the source database and create the VDB from a SnapShot taken after the changes.