Cached file blocks are used only if the corresponding cached attributes are up to date 只要相应缓冲的属性是最新的，缓冲文件块被使用。
2.
Sparse files allow data to be written to random locations within a file without instantiating previously unwritten intervening file blocks 稀疏文件允许把数据写到一个文件的任意位置，而不要将以前未写的中间文件块实例化。
