I had an issue today with ‘df’ and ‘umount’ hanging because of a problem with my Windows (SMB/Samba/CIFS) share. To force unmount I used:
umount -a -t cifs -l
The above command forces all CIFS mounts to unmount immediately.
I was suffering a delay when SSHing to my Debian system. I found SSH session slow to start? It’s the DNS stupid! and solved my problem by editing /etc/ssh/sshd_config and adding: