Distributed File Systems 11.2Process SaiRaj Bharath Yalamanchili

Topics Covered Background Stateful File Service Stateless File Service Advantages and Disadvantages Processes in DFS NFSv4

Background Distributed File System (DFS) A distributed implementation of the classical time-sharing model of a file system, where multiple users share files and storage resources DFS Structure Service – software entity running on one or more machines and providing a particular type of function to a priori unknown clients Server – service software running on a single machine Client – process that can invoke a service using a set of operations that forms its client interface

Stateful File Service Maintains state of the clients. Supports Locking. Effective usage of caching. Fewer disk accesses.

Stateless File Service Does not maintain the state of the clients. Fault Tolerant. Will not guarantee clients whether the request is actually carried or not. Locking is complex.

Advantages/Disadvantages Stateless Servers With stateless server, the effects of server failures and recovery are almost unnoticeable. No need for server to waste time or space maintaining tables of state information Fault tolerant Quick recovery from server crashes Stateful Servers A stateful server loses all its volatile state in a crash. Messages to server are shorter Supports file locking don’t repeat actions if they have been done

Processes in DFS In Most cases, there will be different types of cooperating processes: Storage servers and File managers Are server Stateful or Stateless? NFSv2 and NFSv3 are Stateless NFSv4 is Stateful

Cont. Why Stateless? Server crashes are easy to process since there is no state to recover. The stateless approach in the NFS protocol could not always be fully followed in practical implementations. Locking a file cannot easily be done by a stateless server. In the case of NFS, a separate lock manager is used to handle this situation. The server cannot inform the client whether or not a request has been processed. Likewise, certain authentication protocols require that the server maintains state on its clients.

NFSv4 Maintains some minimal state about its clients; e.g., enough to execute authentication protocols. Stateful servers are better equipped to run over wide area networks, because they are better able to manage consistency issues that arise when clients are allowed to cache portions of files locally. NFS supports callback procedures by which a server can do an RPC to a client. Clearly, callbacks also require a server to keep track of its clients.

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