Barriers – Definition

• In simple terms, we use barriers to enforce ordering of I/O.

• Their primary use is to create a checkpoint in a filesystem journal, ensuring all requests queued ahead of the barrier are complete and reached the physical medium, and any requests queued after it are only started after that point.

• Barriers may or may not be sent with data. A request to the block layer to flush a device is now implemented by sending a barrier I/O without data.

• If a device driver cannot process a barrier it responds with -EOPNOTSUPP and the caller may need to implement the barrier itself.