Debug a large file copy caused system hangs up (for ubuntu)

 use iotop to debug I/O issue

1. # sudo apt-get install iotop

2. # sudo iotop

3. start another terminal

4. # sync

5. to monitor the process to see if it's hanging.

    # top

6. start another terminal, print free memory every 2 seconds

    # free -ms 2

7. Start another terminal, 

# sudo nohup free -ms 2 &> ~/Documents/log/free.log &

# sudo nohup top -b -d 5 &> ~/Documents/log/top.log &

# sudo nohup iotop -b -d 2 -o -t &> ~/Documents/log/iotop.log &

# iostat -x 1           or              collectl -sD

# atop