CSE 542: Operating Systems 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 6-Jul-19 CSE 542: Operating Systems

CSE 60641: Operating Systems 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. 7/6/2019 CSE 60641: Operating Systems

CSE 60641: Operating Systems 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 7/6/2019 CSE 60641: Operating Systems

CSE 60641: Operating Systems 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 7/6/2019 CSE 60641: Operating Systems

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 7/6/2019 CSE 60641: Operating Systems