The df command is useful in reporting disk space usage. However, recently encountered a system that would hang after issuing the df command. A ctl c or ctl z would not escape this annoyance. The issue was quickly resolved by reinstalling stystemd.
yum -y reinstall systemd
After reinstalling systemd, the df command was reissued and was quite responsive.
Source
https://serverfault.com/questions/911811/centos-7-df-started-to-hang