Sometimes, you want to manually alter the caching behaviour of linux. Making sure all data is committed to disk can be done by the sync command. If you want to flush the caches for reads as well, you need to go deeper into the system.

echo 3 > /proc/sys/vm/drop_caches

Writing 1 only clears the pagecache; 2 clears the dentries and inodes; 3 clears all.