root@thinkpad:/home/patrik# echo 3 > /proc/sys/vm/drop_caches
root@thinkpad:/home/patrik# free -m
             total       used       free     shared    buffers     cached
Mem:          3828        481       3346          0          0         79
-/+ buffers/cache:        401       3426
Swap:            0          0          0

root@thinkpad:/home/patrik# echo 3 > /proc/sys/vm/drop_caches
root@thinkpad:/home/patrik# exit
exit

patrik@thinkpad:~$ time ( sync; dd if=/dev/zero of=tempfile bs=1M count=1024 conv=fdatasync,notrunc; sync )
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 1.69178 s, 635 MB/s

real	0m1.931s
user	0m0.012s
sys	0m0.612s
patrik@thinkpad:~$