The class of operations which release locks early are called logical operations. Once such lower-level locks are released, such operations cannot be undone by using the old values of updated data items. Instead, they must be undone by executing a compensating operation called a logical undo operation. In order to allow logical undo of operations, special log records are needed to store the necessary logical undo information. Thus logical undo logging is used widely. Redo operations are performed exclusively using physical log records. This is because the state of the database after a system failure may reflect some updates of an operation and not of other operations, depending on what buffer blocks had been written to disk before the failure. The database state on disk might not be in an operation consistent state, i.e., it might have partial effects of operations. Logical undo or redo operations cannot be performed on an inconsistent data structure.