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Introduction

In a cross-development environment, the Network File System (NFS) client support provided by the SAS/C Connectivity Support Library (CSL) enables the SAS/C Debugger to communicate with the host workstation. This appendix provides the basic information necessary to administer this NFS support. Additional information is contained in Using the NFS Client and SAS Technical Report C-113, SAS/C Connectivity Support Library, Release 1.00.

As an administrator for the SAS/C CSL NFS client, you must be concerned with installing the software, establishing access controls for remote file security, (optionally) developing file-system mount configurations, and diagnosing problems. This support is provided in a distributed file systems environment that uses Sun NFS protocol for network communication between computer systems.

Distributed File Systems

As networking protocols and applications have become more sophisticated, file sharing among computers has evolved from simple file transfer to the construction of distributed file systems. In a distributed file system, programs and users can access (open, read, write, etc.) file systems from a remote machine directly, as if they were attached to the local system.

Although numerous designs for distributed file systems have been implemented experimentally, only a few have achieved commercial success. Of these, the Sun Microsystems Network File System (NFS) protocol is by far the most widely used. Although not as full-featured as some other file systems (most notably the Andrew File System) in areas such as file caching and integrated security administration, its simple and modest design have made it easy to implement on a wide variety of systems. NFS software is currently available for almost every computer and operating system on the market today.

NFS Design

NFS is implemented using a protocol composed of Sun Remote Procedure Call (RPC) function calls. As with most RPC applications, the protocol supports a dialog among servers and clients. The NFS servers are the machines that provide remote access to their file systems. NFS clients are programs that access the files on another system. Use of RPC enhances interoperability among diverse machines.

The NFS protocol views all file systems as conforming to the hierarchical directory organization that has been popularized by the UNIX operating system and that was subsequently codified by the IEEE POSIX standard. The NFS protocol not only allows reading and writing of files, it also supports manipulation of directories.

Each NFS client system builds and maintains its own file system view. This view results from a hierarchical combination of its own file systems and the file systems of servers to which it wants access. At any given directory of this view, the client system may attach a new sub-tree of directories from an NFS server. This process of attaching a new sub-tree of directories is called mounting a remote file system. The directory to which the remote file system is mounted is called the mount point.

An important effect of the mount operation is that the files in the mount-point directory are no longer visible to the client. The newly mounted files in the remote file system are visible instead.

Another important principle is that NFS mounts that are made by a server, when it acts as a client to another system, are not visible to its clients. The clients see only the files that are physically located on the server.

For users of MVS and CMS, perhaps the most important aspect of the NFS design is its orientation toward being a network service instead of being the file system component of a distributed operating system. This orientation is critical in enabling the use of NFS on operating systems that are dissimilar to the UNIX environments in which NFS was originally implemented. The primary requirements for an operating system to participate in NFS are the ability to interpret a hierarchical file system structure and to share UNIX format user identification numbers. Other similarities to UNIX are not required. The SAS/C CSL NFS implementation is able to effect support for directories and UNIX user identification on MVS and CMS.

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