I/O Buffering

Unbuffered I/O

• Data transfered from (to) user directly to (from) device (no intermediate storage)
• Each read or write causes an actual (physical) I/O operation
• run file manager and device handler for each transfer, could cause very high process switching overheads (when transferring characters for instance - possible to get process switch per character)
• Not the best way - clearly - any alternatives ?

Buffered I/O

• No direct transfer from (to) user to (from) I/O device
• Used to smooth peaks in I/O traffic
• Particularly useful for dealing with scenarios where there is a difference in data transfer rates - linking fast devices to slow ones for instance
• Also useful for storing data temporarily ...
• OUTPUT : User's output data transferred to OS output buffer.
  User continues operation, and is only suspended when the buffer becomes full
• INPUT : OS reads ahead for user. Read normally satisfied from buffer. User is blocked if buffer is empty

Double buffering :

  
Figure 8: Double Buffering between OS and User

The Spooler :

  
Figure 9: The Spooler Process