(PECL eio >= 0.0.1dev)
eio_fdatasync — Synchronize a file's in-core state with storage device.
$fd
   [, int $pri = EIO_PRI_DEFAULT
   [, callable $callback = NULL
   [, mixed $data = NULL
  ]]] )eio_fdatasync() synchronizes a file's in-core state with storage device.
fd
    The file descriptor, e.g. returned by eio_open().
pri
    请求的优先级: EIO_PRI_DEFAULT, EIO_PRI_MIN, EIO_PRI_MAX, or NULL.
如果是 NULL , pri 将设为
EIO_PRI_DEFAULT.
callback
    
callback 函数将在请求结束时被调用. 
必须符合此原型: 
void callback($data, $result);
$data 是传给该请求的特定数据.
$result 包含特定请求的结果值, 通常是相应的系统调用的返回值.
    data
    
     Arbitrary variable passed to callback.
     
    eio_fdatasync() returns request resource on success or FALSE on error.