Consistency in NFS and AFS

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

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

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

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

AFS Illustrated Server cache: X Client AClient B

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

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

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

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

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

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

AFS Illustrated Server cache: X Client A cache: XClient B cache: X close X X  X

AFS Illustrated Server cache: X Client A cache: XClient B cache: X close X X  X

AFS Illustrated Server cache: X Client A cache: XClient B cache: X close X

AFS Illustrated Server cache: X Client A cache: XClient B cache: X open X X

AFS Illustrated Server cache: X Client A cache: XClient B cache: X open X X