public static function warnings(array $stores = null) {
        global $CFG;
        if ($stores === null) {
            require_once($CFG->dirroot.'/cache/locallib.php');
            $stores = cache_administration_helper::get_store_instance_summaries();
        }
        $warnings = array();
        foreach ($stores as $store) {
            if (!empty($store['warnings'])) {
                $warnings = array_merge($warnings, $store['warnings']);
            }
        }
        return $warnings;
    }