To clear the disk cache on a linux system you have to issue the following comnad:
sync ; sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
If you are already root, then issue it without the sudo command:
sync ; echo 3 > /proc/sys/vm/drop_caches
The number 3 can be changed with one from the following table:
|2||free dentries and inodes|
|3||free pagecache, dentries and inodes|