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Resolving ORA-01555 Snapshot Too Old during Masking Operations (KBA1145)


Troubleshooting ORA-01555

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 issue 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 is 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 similar to any other configuration option.

If you change the UNDO space on the VDB and then refresh/ rewind it, the operation that was just 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.
  • Create the VDB using a vdb config template that specifies the proper UNDO sizes.