Consistency in NFS and AFS

1. Consistency in NFS and AFS

2. Network File System (NFS) • Uses client caching to reduce network load • Built on top of RPC Server cache: X Client A cache: X Client B cache: X

3. NFS Weak Consistency Protocol • Write-through caching: A write updates the server immediately • Other clients poll the server periodically for changes • No guarantees for multiple writers

4. Andrew File System (AFS) • Developed at CMU • Design principles • Files are cached on each client’s disks • NFS caches only in clients’ memory • Callbacks: The server records who has the copy of a file • Write-back cache on file close. The server then tells all clients that own an old copy. • Session semantics: Updates are only visible on close

5. NFS Summary + Simple and highly portable - May become inconsistent sometimes • Does not happen very often

6. AFS Illustrated Server cache: X Client A Client B

7. read X AFS Illustrated callback list of X client A Server cache: X Client A Client B read X

8. read X AFS Illustrated callback list of X client A Server cache: X Client A cache: X Client B read X

9. read X AFS Illustrated callback list of X client A Server cache: X Client A cache: X Client B read X

10. read X AFS Illustrated callback list of X client A client B Server cache: X Client A cache: X Client B read X

11. read X AFS Illustrated callback list of X client A client B Server cache: X Client A cache: X Client B cache: X read X

12. AFS Illustrated Server cache: X Client A cache: X Client B cache: X write X, X  X

13. X  X AFS Illustrated Server cache: X Client A cache: X Client B cache: X close X

14. X  X AFS Illustrated Server cache: X Client A cache: X Client B cache: X close X

15. AFS Illustrated Server cache: X Client A cache: X Client B cache: X close X

16. X AFS Illustrated Server cache: X Client A cache: X Client B cache: X open X

17. X AFS Illustrated Server cache: X Client A cache: X Client B cache: X open X