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Consistency in NFS and AFS

This article covers the consistency mechanisms in NFS and AFS network file systems, including client caching, write-through caching, callback lists, and session semantics.

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Consistency in NFS and AFS

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  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

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