Managing NFS and NIS, 2nd Edition - Mike Eisler [77]
NFS protocol and implementation
NFS is an RPC-based protocol, with a client-server relationship between the machine having the filesystem to be distributed and the machine wanting access to that filesystem. NFS kernel server threads run on the server and accept RPC calls from clients. These server threads are initiated by an nfsd daemon. NFS servers also run the mountd daemon to handle filesystem mount requests and some pathname translation. On an NFS client, asynchronous I/O threads (async threads) are usually run to improve NFS performance, but they are not required.
On the client, each process using NFS files is a client of the server. The client's system calls that access NFS-mounted files make RPC calls to the NFS servers from which these files were mounted. The virtual filesystem really just extends the operation of basic system calls like read( ) and write( ), similar to the way that NIS extends the operation of library calls like getpwuid( ). In NIS, the getpwuid( ) routine knows how to use the NIS RPC protocol to locate user information that isn't in the local /etc/passwd file. Within the virtual filesystem, the basic file- and filesystem-oriented system calls were modified to "know" how to operate on non-local filesystems.
Let's look at this with an example. On an NFS client, a user process executes a chmod( ) system call on an NFS-mounted file. The virtual filesystem passes this system call to NFS, which then executes a remote procedure call to set the permissions on the file, as specified in the process's system call. When the RPC completes, the system call returns to the user process. This example is fairly simple, because it doesn't involve any block I/O to get file data to or from the NFS server. When blocks of files are moved around, the async threads get involved to improve NFS performance. This section covers the protocols used by NFS and features of its implementation that were driven by performance or transparency goals.
NFS RPC procedures
Each version of the NFS RPC protocol contains several procedures, each of which operates on either a file or a filesystem object. The basic procedures performed on an NFS server can be grouped into directory operations, file operations, link operations, and filesystem operations. Directory operations include mkdir and rmdir, which create and destroy directories like their Unix system call equivalents. readdir reads a directory, using an opaque directory pointer to perform sequential reads of the same directory. Other directory-oriented procedures are rename and remove, which operate on entries in a directory the same way the mv and rm commands do. create makes a new directory entry for a file.
The lookup operation is the heart of the pathname-to-filehandle translation mechanism. lookup finds a named directory entry and returns a filehandle pointing to it. The open( ) system call uses lookup( ) extensively: it breaks a pathname down into its components and locates each component in its parent directory. For example, open( ) would handle the pathname /home/thud/stern by performing three operations:
Look up home in the root directory (/).
Look up thud in /home.
Look up stern in /home/thud.
File operations are very closely associated with Unix system calls: read and write move data to and from the NFS client, and getattr and setattr get or modify the file's attributes. In a local filesystem, such as UFS, these attributes are stored in the file's inode, but file attributes are mapped to whatever system is used by the NFS server. Link operations include link, which