It should be much more safer to sync the data to disk before clear the caches.
sync
1) Clear PageCache only.
echo 1 > /proc/sys/vm/drop_caches
2) Clear dentries and inodes.
echo 2 > /proc/sys/vm/drop_caches
3) Clear PageCache, dentries and inodes.
echo 3 > /proc/sys/vm/drop_caches