buffer&cache

• A buffer is something that has yet to be "written" to disk. 
  
• A cache is something that has been "read" from the disk and stored for later use.

也就是说buffer是用于存放要输出到disk（块设备）的数据的，而cache是存放从disk上读出的数据。

这二者是为了提高IO性能的，并由OS管理。