File system cache / drop cache(debian)

view story

http://serverfault.com – if i play those command : sync echo 3 > /proc/sys/vm/drop_caches Will the server be force to read the correct file on disk until i put 0 again ? Or this work only one time, and if i want to clear cache again, i've to put 0 and 3 again ? Thanks (HowTos)