[collectd] Notification issues

[Sebastiano Pilla]

I have tried every combination of Host, Plugin, Type and Instance that I 
could think of, but still I don't seem to be able to configure a simple 
notification. I've seen the types.db file, but I have yet to understand 
the relationship between

df  used:GAUGE:0:1125899906842623, free:GAUGE:0:1125899906842623

and what I should put into Type or Instance or DataSource.

So, given this configuration:

LoadPlugin df
<Plugin "df">
   MountPoint "/"
   MountPoint "/boot"
</Plugin>
LoadPlugin exec
<Plugin exec>
   NotificationExec root "/etc/collectd/collectd-notify.sh"
</Plugin>
LoadPlugin threshold
<Threshold>
...
</Threshold>

can anyone fill in the blanks in the Threshold configuration with an 
example of sending a notification when the free space on either the "/" 
or the "/boot" mount point goes below X percent ?

Thanks in advance
Sebastiano Pilla