private double diskOverhead(double hostAdvertisedDiskGb, NodeResources resources, boolean real) {
            double diskShare = resources.diskGb() /
                               ( hostAdvertisedDiskGb - (real ? diskOverhead : 0) );
            return diskOverhead * diskShare;
        }
Returns the disk overhead resulting if the given advertised resources are placed on the given node

@param real true if the resources are in real values, false if they are in advertised
node-repository/src/test/java/com/yahoo/vespa/hosted/provision/provisioning/VirtualNodeProvisioningCompleteHostCalculatorTest.java