Sometimes, we have such a situation happening with droping the OS cache. We may usually tell the OS to use direct io or unbuffered reads and writes. How to drop the OS caches?
To use /proc/sys/vm/drop_caches, just echo a number to it.
To free pagecache:
# echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
# echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches
This is a non-destructive operation and will only free things that are completely unused. Dirty objects will continue to be in use until written out to disk and are not freeable.