Update all of the enabled fileserver backends which support the update
        function, or

def update(self, back=None):
        '''
        Update all of the enabled fileserver backends which support the update
        function, or
        '''
        back = self.backends(back)
        for fsb in back:
            fstr = '{0}.update'.format(fsb)
            if fstr in self.servers:
                log.debug('Updating %s fileserver cache', fsb)
                self.servers[fstr]()