Monitoring disk space on OpenVZ container

Hi,

   I would like to monitor the disk space on a virtual OpenVZ machine. 
Unfortunately, these error comes up on the XYmon webpage : "Expected 
strings (Capacity and Mounted) not found in df output"

   On the server :
   ~# df
    Filesystem           1K-blocks      Used Available Use% Mounted on
    /dev/simfs            26214400    644940  25569460   3% /
    tmpfs                   262144         0    262144   0% /lib/init/rw
    tmpfs                   262144         0    262144   0% /dev/shm

   Do you have a way to monitor the / filesystem ?


   Thanks