public static function _clear()
    {
        if (self::getBackend() == 'memcache') {
            if (!Kwf_Config::getValue('cacheSimpleNamespace')) {
                //increment namespace
                $mc = Kwf_Cache_Simple::getMemcache();
                if ($mc->get(Kwf_Cache_Simple::$uniquePrefix.'cache_namespace')) {
                    $mc->increment(Kwf_Cache_Simple::$uniquePrefix.'cache_namespace');
                }
            } else {
                $mc = Kwf_Cache_Simple::getMemcache();
                $mc->flush();
            }
        } else if (self::getBackend() == 'redis') {
            $prefixLength = strlen(self::getRedis()->_prefix(''));
            $it = null;
            while ($keys = self::getRedis()->scan($it, self::getRedis()->_prefix('simple:*'))) {
                foreach ($keys as $k=>$i) {
                    $keys[$k] = substr($i, $prefixLength);
                }
                self::getRedis()->delete($keys);
            }
        } else if (self::getBackend() == 'file') {
            foreach(glob('cache/simple/*') as $i) {
                unlink($i);
            }
        } else if (self::getBackend() == 'apc' || self::getBackend() == 'apcu') {
            //those are cleared using their own clear-cache type
        } else {
            if (!isset(self::$_zendCache)) self::getZendCache();
            $r = self::$_zendCache->clean(Zend_Cache::CLEANING_MODE_ALL);
        }
    }