slight enhancement to 'check device'

[Mike]

Reading the news group archive on gmane I have the example:

> check device rootfs with path /dev/hda1                                       
>                                          
>       if space usage > 80% then alert                                         
>                                          
>       mode passive                                                            
>                                          

Is there a way to monitor how fast a filespace is growing in
addition to the absolute space used in the filespace? Knowing
that the filespace is only 80% full, however since the last
time the monitor check the filespace has grown 30%. Though
there is sufficient space at the moment, still 20% to go, by
the next monitoring interval at the current rate of growth
the filesystem will be full.

Mike