dddd if=/dev/zero of=./test bs=512k count=2048 oflag=direct
sudo /sbin/sysctl -w vm.drop_caches=3
Don't worry about resetting this flag. The cache is only dropped when
sysctlis ran.
dd if=./test of=/dev/zero bs=512k count=2048
source: https://www.unixtutorial.org/test-disk-speed-with-dd/