So I started investigating what was the root cause. Found a huge log file that is 3.3G in size - amUpgrade.
I then executed a tail command on the file. Wow! It was moving every single milli-second.
Just found out this was reported in bugster OPENAM-1791 - When in message level debug, amUpgrade file logs entries on every request to OpenAM console.
Time to patch your OpenAM if your are on the affected version. Workaround is not to turn on MESSAGE debug level, unless required.