76 Distributed Filesystems Probably the first thing we are interested in doing

76 distributed filesystems probably the first thing

This preview shows page 85 - 87 out of 102 pages.

7.6 Distributed Filesystems Probably the first thing we are interested in doing with a network is making our files available to all hosts, so that - no matter where in our corporate empire we happen to be sitting - we always have access to our files. The concept of a distributed filesystem is about sharing disks across a network. Many operating systems have There are three main contenders for such a system in the UNIX world. Only one of these is in widespread use. 7.6.1 NFS - the network filesystem NFS was historically the first distributed filesystem to be implemented, by Sun Microsystems. All manufacturers now support Sun's NFS.
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NFS is based on Sun's own RPC system (Remote procedure call). The idea behind NFS is to imitate UNIX filesystem semantics as closely as possible from across a network. NFS works by implementing a number of servers which run on UNIX machines. One problem with a network file system is what to do about machine crashes. Suppose we are in the middle of writing or retrieving a file and the server machine supplying the file crashes. We need some way of remembering where we were, so that when the machine comes back up, the operation can continue where it left off. In fact this is almost impossible to achieve in practice - NFS's solution works in many cases, but not in all. In the UNIX filesystem, a user must obtain a lock on a file in order to read or write to it. In NFS, the same system applies. A lock is obtained from a lock server on the host where the real disk filesystem lies and the state of the filesystem is communicated by a state server . NFS is sometimes called a stateless protocol, but this is a misleading title. The state of the filesystem on the server is maintained on the server which owns the filesystem. If there is a crash, the server tries to reestablish the locks it held before the crash. If this is not possible because the filesystem has changed in the meantime or because of unfortunate timing, the result is a `stale NFS filehandle' - an unrecoverable error. The state information has to be cleared and restarted. NFS is called stateless because the server does not record the requests which the client makes (except for locks). The server processes requests without caring about which client it is serving, or about what it has done in previous requests. It doesn't know how much of a file the client has read. In other words, it is the client's responsibility to keep track of what requests it sends to the server and whether or not it gets a reply. NFS version 3 is now in use by some vendors and includes a number of improvements (and a few new bugs) over NFS. These include better caching, access control lists (ACLs) etc. 7.6.2 AFS - the andrew filesystem Another filesystem which is increasingly discussed, is the Andrew file system. The CERN high energy physics (HEP) group use the AFS as a global filesystem and many other institutions are starting to follow suit. Whereas NFS tries to reproduce UNIX-like file semantics across a network, AFS is a different filesystem altogether. AFS solves the problem of user authentification between different sites. A problem in sharing
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