1. DISTRIBUTED FILE SYSTEM- ENHANCEMENT AND FURTHER DEVELOPMENT BY:- PALLAWI(10BIT0033)

2. Distributed File System  A distributed file system is a client/server based application that allows clients to access and process data stored on the server.  Sun Microsystems' Network File System (NFS), Novell NetWare, Microsoft's Distributed File System, and IBM/Transarc's DFS are some examples of distributed file systems.

3. Enhancement  Several advances have been made in the design of distributed file systems since the emergence of NFS and AFS.  Advances that enhance the performance, availability and scalability of conventional distributed file systems.

4. Enhancement Includes…  NFS enhancements  AFS enhancements  Improvement in storage organization  New design approaches

5. NFS Enhancements  Several research projects have addressed the need for one-copy update semantics by extending the NFS protocol to include open and close operations and adding a callback mechanism to enable the server to notify clients of the need to invalidate cache entries.  NFS developer have been directed at making NFS servers more accessible and useful in wide-area networks.

6. NFS enhancement includes… Achieving one-copy update semantics. NQNFS WebNFS NFS version 4

7. Achieving one-copy update semantics. The stateless server architecture of NFS brought great advantages in terms of robustness and ease of implementation. It prevents the achievement of precise one- copy update semantics. It also prevents the use of callbacks.

8. Contd… Research systems have been developed that address these drawbacks. Spritely NFS:- implementation of the NFS protocol with the addition of open and close calls.

9. Spritely NFS Clients’ modules must send an open operation on the server. The parameters of the Sprite open operation specify a mode(read, write or both) and include counts of the number of local processes. when a local process closes a remote file, a close operation is sent to the server with updated counts of readers and writers.

10. Contd… The server records these numbers in an open files table with the IP address and port number of the client. When the server receives an open, it checks the open files table for other clients that have the same file open and sends callback messages to those clients instructing them to modify their caching strategy. If the open specifies write mode, then it will fail if any other client has the file open for writing. For open operations that specify read mode, the server sends a callback message to any client that is writing, instructing it to stop caching

11. Spritely NFS implements a recovery protocol that interrogates a list of clients that have recently opened files on the server to recover the full open files table

12. NQNFS Aim to add more precise cache consistency to the NFS protocol and to improve performance through better use of caching. It maintains similar client-related state concerning open files, but it uses leases to aid recovery after a server crash. The server sets an upper bound on the time If the client wishes to continue beyond that time, it must renew the lease. if the clients don’t reply, the server simply waits until their leases expire before responding to the new write request.

13. WebNFS To enable web browsers and other applications to access files on an NFS server that ‘publishes’ them using a public file handle relative to a public root directory. WebNFS clients interact with an NFS server at a well-known port number(2049). To access files by pathname, they issue lookup requests using a public file handle. The public file handle has a well-known value that is interpreted specially by the virtual file system at the server.

14. Contd… Because of the high latency of wide-area networks, a multicomponent variant of the lookup operation is used to look up a multi- part pathname in a single request. Thus WebNFS enables clients to be written that access portions of files stored in NFS servers at remote sites with minimal setup overheads.

15. NFS version 4 It aims to make it practical to use NFS in wide- area networks and Internet applications. It includes the features of WebNFS. NFS version 4 exploits results such as the use of callbacks or leases to maintain consistency. NFS version 4 supports on-the-fly recovery from server faults. Scalability is improved by using proxy servers.

16. AFS enhancements In AFS, callbacks are generated only when the server receives a close operation for a file that has been updated. In order to update a file, a client must obtain a write token from the server, specifying a range of bytes in the file that the client is permitted to update. When a write token is requested, clients holding copies of the same file for reading receive revocation callbacks. All tokens have an associated lifetime, and clients must renew them after their lifetime has expired.

17. Improvements in storage organization There has been considerable progress in the organization of file data stored on disks. The principal results of this work are: Redundant Arrays of Inexpensive Disks (RAID):This is a mode of storage in which data blocks are segmented into fixed-size chunks and stored in ‘stripes’ across several disks, along with redundant error-correcting codes that enable the data blocks to be reconstructed completely and operation to continue normally in the event of disk failures. Log-structured file storage (LFS):

18. The authors observed that as larger amounts of main memory became available for caching in file servers, an increased level of cache hits resulted in excellent read performance, but write performance remained mediocre. The LFS solution is to accumulate a set of writes in memory and then commit them to disk in large, contiguous, fixed-sized segments. These are called log segments because the data and metadata blocks are stored strictly in the order in which they were updated.

19. New design approaches xFS:- Their approach was motivated by three factors: 1.the opportunity provided by fast switched LANs for multiple file servers in a local network to transfer bulk data to clients concurrently; 2. increased demand for access to shared data; 3. the fundamental limitations of systems based on central file servers.