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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: X. Client B cache: X. NFS Weak Consistency Protocol. Write-through caching: A write updates the server immediately

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network file system nfs
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

nfs weak consistency protocol
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
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
NFS Summary

+ Simple and highly portable

- May become inconsistent sometimes

  • Does not happen very often
afs illustrated
AFS Illustrated

Server cache: X

Client A

Client B

afs illustrated1
read XAFS Illustrated

callback list of X

client A

Server cache: X

Client A

Client B

read X

afs illustrated2
read XAFS Illustrated

callback list of X

client A

Server cache: X

Client A cache: X

Client B

read X

afs illustrated3
read XAFS Illustrated

callback list of X

client A

Server cache: X

Client A cache: X

Client B

read X

afs illustrated4
read XAFS Illustrated

callback list of X

client A

client B

Server cache: X

Client A cache: X

Client B

read X

afs illustrated5
read XAFS Illustrated

callback list of X

client A

client B

Server cache: X

Client A cache: X

Client B cache: X

read X

afs illustrated6
AFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

write X, X  X

afs illustrated7
X  XAFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

close X

afs illustrated8
X  XAFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

close X

afs illustrated9
AFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

close X

afs illustrated10
XAFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

open X

afs illustrated11
XAFS Illustrated

Server cache: X

Client A cache: X

Client B cache: X

open X

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