Consistency in NFS and AFS

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Consistency in NFS and AFS
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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
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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

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Andrew File System (AFS)

• Developed at CMU
• Design principles
• Files are cached on each clients 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

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NFS Summary

• Simple and highly portable
• - May become inconsistent sometimes
• Does not happen very often

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AFS Illustrated
Server cache X
Client A
Client B
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AFS Illustrated
callback list of X client A
Server cache X
Client A
Client B
read X
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AFS Illustrated
callback list of X client A
Server cache X
Client A cache X
Client B
read X
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AFS Illustrated
callback list of X client A
Server cache X
Client A cache X
Client B
read X
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AFS Illustrated
callback list of X client A client B
Server cache X
Client A cache X
Client B
read X
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AFS Illustrated
callback list of X client A client B
Server cache X
Client A cache X
Client B cache X
read X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
write X, X ? X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
close X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
close X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
close X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
open X
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AFS Illustrated
Server cache X
Client A cache X
Client B cache X
open X