回调的 API 异步地执行所有操作，不会阻塞事件循环，然后在完成或错误时调用回调函数。
The callback APIs perform all operations asynchronously, without blocking the event loop, then invoke a callback function upon completion or error.
The callback APIs use the underlying Node.js threadpool to perform file system operations off the event loop thread. These operations are not synchronized or threadsafe. Care must be taken when performing multiple concurrent modifications on the same file or data corruption may occur.