Bulletproof web design - Inside Linux Basically, the NFS server exports a
Inside Linux Basically, the NFS server exports a file or directory, making it visible to NFS clients. The NFS client simply mounts the requested filesystem. The mechanism of mount is that of incorporating the remote filesystem with the local filesystem. A specialized service that is available for the client side is the automounter. The automounter provides the capability to automatically mount and umount (unmount) a filesystem when that filesystem is required. If you are familiar with the mtools package, consisting of commands such as mdir and mcopy, you are familiar with automount functionality. When you insert a DOS floppy and execute the mdir command, the command first mounts the floppy, and then accesses the floppy’s directory structure through the local (mounted) directory, and finally, mdir executes umount on the floppy. NFS Protocol The NFS protocol is composed of remote procedures that provide clients the capability to manipulate files and directories on remote machines (NFS servers) as if those files were local. The NFS protocol provides the routines necessary for a client to send a request for a file or directory operation. The server accepts this request and attempts to execute the request. Whether the request succeeds or fails, a response is returned to the client making the request. In the case of failure, an error value is returned. In the case of success, the results appropriate for the execution of the request are returned to the client. Various file operations are available in the NFS protocol, such as read, rename, mkdir (make directory), and write. Using the NFS protocol routines, clients send requests for file operations to servers, and servers respond by attempting to perform the operation (provided the user has proper permission) and then sending back successful results or error values. The NFS protocol includes a full spectrum of file operations, including mkdir, read, rename, and write. Error Recovery One of the design goals for the NFS protocol is swift error recovery. Each of the NFS calls identifies the arguments necessary to execute that procedure call. This clearly provides two benefits of NFS. The first benefit is that NFS can gracefully and quickly recover from any failure, be it network or system. The second benefit allows for painless administration. Transport Independence NFS is not associated with any transport protocol. This results in a file-sharing system that is transport independent. The NFS architecture is designed so that any future protocols can carry NFS traffic. A few protocols are in use today, such as User Datagram Protocol (UDP) and Transmit Control Protocol (TCP). Protocol independence is achieved using a Remote Procedure Call (RPC) layer; the NFS protocol is actually implemented on top of the RPC transport technology. This implies that the NFS calls are implemented in terms of RPC calls. NFS also utilizes the External Data Representation (XDR), which further aids in NFS being machine and language independent. XDR is a standard for the description and encoding of data. It is used for transferring data between disparate computer architectures. A language is used to describe data formats in XDR - it can be used only to describe data. NFS Client An NFS server exports files and directories, thereby allowing NFS clients to gain access to those resources. The NFS client simply mounts the remote filesystem to a local resource, such as a directory. This results in the remote filesystems being an integral part of the local Linux filesystem. Figure 12.1 visually demonstrates a mounted filesystem. A service called the automounter provides functionality to automatically mount a filesystem and then unmount it when an operation is complete. The automounter is an NFS client-side enhancement, reducing the manual tasks of mounting and unmounting a remote filesystem. page 212
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