Unable to start OpenVZ VPS: Failed to umount /vz/root/1556: Device or resource busy [22]

Symptoms 

Unable to start OpenVZ VPS:

# vzctl start 1556
Starting container...
Unmounting file system at /vz/root/1556
--- lsof /vz/root/1556 output ---
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
find 722326 root cwd DIR 182,315505 4096 2359767 /vz/root/1556/home/admin/web/phongxodiax.com/public_html/wp-content/cache/all/calorie-chart/calorie-in-a-dessert/1200
find 722326 root 5r DIR 182,315505 4096 2359767 /vz/root/1556/home/admin/web/phongxodiax.com/public_html/wp-content/cache/all/calorie-chart/calorie-in-a-dessert/1200
--- lsof finished ---
Error in do_umount (ploop.c:992): Failed to umount /vz/root/1556: Device or resource busy
Failed to umount image: Error in do_umount (ploop.c:992): Failed to umount /vz/root/1556: Device or resource busy [22]

Cause

The files from the error are used by another process.

Resolution

1. Connect to OpenVZ node via SSH
2. Kill the process with the command:

   # kill -9 722326