The algorithm behind the incremental backups searches for those modified blocks whose block System Change Number (SCN) is higher than the last incremental level backup's Incremental Start SCN. Though it's a very good workaround to skip those unchanged blocks which we don't need to back up again now, to find the list of the candidate changed blocks, it would be required to read the entire data file even when the backup type is going to be an incremental backup. That's why although the incremental backups are faster than the comparative full backups, they are not really fast enough!
Note
An SCN within Oracle database is the internal representation of time. Using an SCN, a database keeps track of when something happened. For transactions, SCN is incremented within a block with every commit. But an SCNs are used at many different places within the database. The complete discussion about SCNs is beyond the scope of this book....
