wordpress errors

Hello all. Every once in a while my website gives me the following errors:

Warning: Unknown(): open(/tmp/sess_e04cc701df9e723addd7276ff4105cce, O_RDWR) failed: No space left on device (28) in Unknown on line 0

Warning: Unknown(): Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0

I’m not too sure if it is the same errors every time, but those are the current ones.

Any reason as to why?

Thank you,
Andrew.
http://www.beleafer.ca