show how to analyse per node disk usage

tsa/howto/ganeti.mdwn

@@ -149,6 +149,11 @@ memory, and compare it with the node's capacity:
 
     watch -n5 -d 'gnt-instance list -o pnode,name,be/vcpus,be/memory,status,disk_template  |  sort; echo; gnt-node list'
 
+The latter does not show disk usage for secondary volume groups, for a
+complete picture of disk usage, try this:
+
+    gnt-cluster command vgs
+
 The [gnt-cluster verify](http://docs.ganeti.org/ganeti/2.15/man/gnt-cluster.html#verify) command will also check to see if there's
 enough space on secondaries to account for the failure of a
 node. Healthy output looks like this: