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Distributed systems

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  1. Distributed systems • Spritely NFS: experiments with cache-consistency protocols. Srinivasan, V. and Mogul, J. SOSP '89 • Distributed systems: large overhead between client and server • Maintaining cache between clients and server is a big challenge: whole file vs block based caching, stateless server vs stateful server, and cache consistency models CSE 542: Operating Systems

  2. NFS • Stateless server: clients submit all requests parameters with each request • Write-through: clients cannot leave writes in cache because if clients fail, write is lost • Block cache: not whole file, periodic check with svr • Easy crash recovery: if server crashes, the system is down while the server is down but will come back just fine. If client crashes, nothing is lost because all writes are write-through. • Different than UFS consistency: UNIX lets you operate on open files (even if deleted later). Since NFS is stateless, all requests might be validated. CSE 60641: Operating Systems

  3. Sprite DFS • Stateful server: cached in client and server. Server knows who has opened a file (open and close are sent to the server). Single writer files need not do write-through, might never write through if file was deleted • Write-back: If concurrently write-opened file, then server disables client cache. Writers become write-through and reads also go through to servers (more stricter than NFS) • Callbacks: use callbacks from server to clients • Guarantee consistency: as much as a local FS • Crash recovery is difficult – especially detecting CSE 60641: Operating Systems

  4. Spritely NFS • Goal: Bring the best aspects of Sprite to NFS (NFS is simple but slow, Sprite is fast) • New system calls: • Open, close to server. No need for getattr (open returns it) • Callback: • All dirty blocks should be written back to server • Cached blocks are invalidated and disable cache • Issues with multithreading client as well as timeouts at the server CSE 60641: Operating Systems

  5. Implementation issues • Client side: • Cache enabled, no change • Open and close are sent to servers, close can be delayed • UNIX read ahead is disabled for non-cacheable files • Call back • Delayed write policy: UNIX like 30 seconds • Server Side: State maintenance CSE 60641: Operating Systems

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