Primary File Organization in DBMS - Files of ordered records (sorted files)

"Fundamental of Database Systems", 3rd ed. by Elmasri and Navathe, page 136 says: "We can physically order the records of a file on disk based on the values of one of their fields [...]" How could this "physical ordering" possibly enforced, especially with regard to fragmentation? Most databases use system calls to store their data in the file system of the OS, only very few are capable of doing raw i/o.