1. 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

2. 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

3. 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

4. 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

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
7/6/2019
CSE 60641: Operating Systems