Книга: Distributed operating systems

5.2.5. An Example: Sun's Network File System

5.2.5. An Example: Sun's Network File System

In this section we will examine an example network file system, Sun Microsystem's Network File System, universally known as NFS. NFS was originally designed and implemented by Sun Microsystems for use on its UNIX-based workstations. Other manufacturers now support it as well, for both UNIX and other operating systems (including MS-DOS). NFS supports heterogeneous systems, for example, MS-DOS clients making use of UNIX servers. It is not even required that all the machines use the same hardware. It is common to find MS-DOS clients running on Intel 386 CPUs getting service from UNIX file servers running on Motorola 68030 or Sun SPARC CPUs.

Three aspects of NFS are of interest: the architecture, the protocol, and the implementation. Let us look at these in turn.

NFS Architecture

The basic idea behind NFS is to allow an arbitrary collection of clients and servers to share a common file system. In most cases, all the clients and servers are on the same LAN, but this is not required. It is possible to run NFS over a wide-area network. For simplicity we will speak of clients and servers as though they were on distinct machines, but in fact, NFS allows every machine to be both a client and a server at the same time.

Each NFS server exports one or more of its directories for access by remote clients. When a directory is made available, so are all of its subdirectories, so in fact, entire directory trees are normally exported as a unit. The list of directories a server exports is maintained in the /etc/exports file, so these directories can be exported automatically whenever the server is booted.

Clients access exported directories by mounting them. When a client mounts a (remote) directory, it becomes part of its directory hierarchy, as shown in Fig. 5-13. Many Sun workstations are diskless. If it so desires, a diskless client can mount a remote file system on its root directory, resulting in a file system that is supported entirely on a remote server. Those workstations that do have local disks can mount remote directories anywhere they wish on top of their local directory hierarchy, resulting in a file system that is partly local and partly remote. To programs running on the client machine, there is (almost) no difference between a file located on a remote file server and a file located on the local disk.

Thus the basic architectural characteristic of NFS is that servers export directories and clients mount them remotely. If two or more clients mount the same directory at the same time, they can communicate by sharing files in their common directories. A program on one client can create a file, and a program on a different one can read the file. Once the mounts have been done, nothing special has to be done to achieve sharing. The shared files are just there in the directory hierarchy of multiple machines and can be read and written the usual way. This simplicity is one of the great attractions of NFS.

NFS Protocols

Since one of the goals of NFS is to support a heterogeneous system, with clients and servers possibly running different operating systems on different hardware, it is essential that the interface between the clients and servers be well defined. Only then is it possible for anyone to be able to write a new client implementation and expect it to work correctly with existing servers, and vice versa.

NFS accomplishes this goal by defining two client-server protocols. A protocol is a set of requests sent by clients to servers, along with the corresponding replies sent by the servers back to the clients. (Protocols are an important topic in distributed systems; we will come back to them later in more detail.) As long as a server recognizes and can handle all the requests in the protocols, it need not know anything at all about its clients. Similarly, clients can treat servers as "black boxes" that accept and process a specific set of requests. How they do it is their own business.

The first NFS protocol handles mounting. A client can send a path name to a server and request permission to mount that directory somewhere in its directory hierarchy. The place where it is to be mounted is not contained in the message, as the server does not care where it is to be mounted. If the path name is legal and the directory specified has been exported, the server returns a file handle to the client. The file handle contains fields uniquely identifying the file system type, the disk, the i-node number of the directory, and security information. Subsequent calls to read and write files in the mounted directory use the file handle.

Many clients are configured to mount certain remote directories without manual intervention. Typically, these clients contain a file called /etc/rc, which is a shell script containing the remote mount commands. This shell script is executed automatically when the client is booted.

Alternatively, Sun's version of UNIX also supports automounting. This feature allows a set of remote directories to be associated with a local directory. None of these remote directories are mounted (or their servers even contacted) when the client is booted. Instead, the first time a remote file is opened, the operating system sends a message to each of the servers. The first one to reply wins, and its directory is mounted.

Automounting has two principal advantages over static mounting via the /etc/rc file. First, if one of the NFS servers named in /etc/rc happens to be down, it is impossible to bring the client up, at least not without some difficulty, delay, and quite a few error messages. If the user does not even need that server at the moment, all that work is wasted. Second, by allowing the client to try a set of servers in parallel, a degree of fault tolerance can be achieved (because only one of them need to be up), and the performance can be improved (by choosing the first one to reply — presumably the least heavily loaded).

On the other hand, it is tacitly assumed that all the file systems specified as alternatives for the automount are identical. Since NFS provides no support for file or directory replication, it is up to the user to arrange for all the file systems to be the same. Consequently, automounting is most often used for read-only file systems containing system binaries and other files that rarely change.

The second NFS protocol is for directory and file access. Clients can send messages to servers to manipulate directories and to read and write files. In addition, they can also access file attributes, such as file mode, size, and time of last modification. Most UNIX system calls are supported by NFS, with the perhaps surprising exception of OPEN and CLOSE.

The omission of OPEN and CLOSE is not an accident. It is fully intentional. It is not necessary to open a file before reading it, nor to close it when done. Instead, to read a file, a client sends the server a message containing the file name, with a request to look it up and return a file handle, which is a structure that identifies the file. Unlike an OPEN call, this LOOKUP operation does not copy any information into internal system tables. The READ call contains the file handle of the file to read, the offset in the file to begin reading, and the number of bytes desired. Each such message is self-contained. The advantage of this scheme is that the server does not have to remember anything about open connections in between calls to it. Thus if a server crashes and then recovers, no information about open files is lost, because there is none. A server like this that does not maintain state information about open files is said to be stateless.

In contrast, in UNIX System V, the Remote File System (RFS) requires a file to be opened before it can be read or written. The server then makes a table entry keeping track of the fact that the file is open, and where the reader currently is, so each request need not carry an offset. The disadvantage of this scheme is that if a server crashes and then quickly reboots, all open connections are lost, and client programs fail. NFS does not have this property.

Unfortunately, the NFS method makes it difficult to achieve the exact UNIX file semantics. For example, in UNIX a file can be opened and locked so that other processes cannot access it. When the file is closed, the locks are released. In a stateless server such as NFS, locks cannot be associated with open files, because the server does not know which files are open. NFS therefore needs a separate, additional mechanism to handle locking.

NFS uses the UNIX protection mechanism, with the rwx bits for the owner, group, and others. Originally, each request message simply contained the user and group ids of the caller, which the NFS server used to validate the access. In effect, it trusted the clients not to cheat. Several years' experience abundantly demonstrated that such an assumption was — how shall we put it? — naive. Currently, public key cryptography can be used to establish a secure key for validating the client and server on each request and reply. When this option is enabled, a malicious client cannot impersonate another client because it does not know that client's secret key. As an aside, cryptography is used only to authenticate the parties. The data themselves are never encrypted.

All the keys used for the authentication, as well as other information are maintained by the NIS (Network Information Service). The NIS was formerly known as the yellow pages. Its function is to store (key, value) pairs. when a key is provided, it returns the corresponding value. Not only does it handle encryption keys, but it also stores the mapping of user names to (encrypted) passwords, as well as the mapping of machine names to network addresses, and other items.

The network information servers are replicated using a master/slave arrangement. To read their data, a process can use either the master or any of the copies (slaves). However, all changes must be made only to the master, which then propagates them to the slaves. There is a short interval after an update in which the data base is inconsistent.

NFS Implementation

Although the implementation of the client and server code is independent of the NFS protocols, it is interesting to take a quick peek at Sun's implementation. It consists of three layers, as shown in Fig. 5-14. The top layer is the system call layer. This handles calls like OPEN, READ, and CLOSE. After parsing the call and checking the parameters, it invokes the second layer, the virtual file system (VFS) layer.


Fig. 5-14. NFS layer structure.

The task of the VFS layer is to maintain a table with one entry for each open file, analogous to the table of i-nodes for open files in UNIX. In ordinary UNIX, an i-node is indicated uniquely by a (device, i-node number) pair. Instead, the VFS layer has an entry, called a v-node (virtual i-node), for every open file. V-nodes are used to tell whether the file is local or remote. For remote files, enough information is provided to be able to access them.

To see how v-nodes are used, let us trace a sequence of MOUNT, OPEN, and READ system calls. To mount a remote file system, the system administrator calls the mount program specifying the remote directory, the local directory on which it is to be mounted, and other information. The mount program parses the name of the remote directory to be mounted and discovers the name of the machine on which the remote directory is located. It then contacts that machine asking for a file handle for the remote directory. If the directory exists and is available for remote mounting, the server returns a file handle for the directory. Finally, it makes a MOUNT system call, passing the handle to the kernel.

The kernel then constructs a v-node for the remote directory and asks the NFS client code in Fig. 5-14 to create an r-node (remote i-node) in its internal tables to hold the file handle. The v-node points to the r-node. Each v-node in the VFS layer will ultimately contain either a pointer to an r-node in the NFS client code, or a pointer to an i-node in the local operating system (see Fig. 5-14). Thus from the v-node it is possible to see if a file or directory is local or remote, and if it is remote, to find its file handle.

When a remote file is opened, at some point during the parsing of the path name, the kernel hits the directory on which the remote file system is mounted. It sees that this directory is remote and in the directory's v-node finds the pointer to the r-node. It then asks the NFS client code to open the file. The NFS client code looks up the remaining portion of the path name on the remote server associated with the mounted directory and gets back a file handle for it. It makes an r-node for the remote file in its tables and reports back to the VFS layer, which puts in its tables a v-node for the file that points to the r-node. Again here we see that every open file or directory has a v-node that points to either an r-node or an i-node.

The caller is given a file descriptor for the remote file. This file descriptor is mapped onto the v-node by tables in the VFS layer. Note that no table entries are made on the server side. Although the server is prepared to provide file handles upon request, it does not keep track of which files happen to have file handles outstanding and which do not. When a file handle is sent to it for file access, it checks the handle, and if it is valid, uses it. Validation can include verifying an authentication key contained in the RPC headers, if security is enabled.

When the file descriptor is used in a subsequent system call, for example, read, the VFS layer locates the corresponding v-node, and from that determines whether it is local or remote and also which i-node or r-node describes it.

For efficiency reasons, transfers between client and server are done in large chunks, normally 8192 bytes, even if fewer bytes are requested. After the client's VFS layer has gotten the 8K chunk it needs, it automatically issues a request for the next chunk, so it will have it should it be needed shortly. This feature, known as read ahead, improves performance considerably.

For writes an analogous policy is followed. If a WRITE system call supplies fewer than 8192 bytes of data, the data are just accumulated locally. Only when the entire 8K chunk is full is it sent to the server. However, when a file is closed, all of its data are sent to the server immediately.

Another technique used to improve performance is caching, as in ordinary UNIX. Servers cache data to avoid disk accesses, but this is invisible to the clients. Clients maintain two caches, one for file attributes (i-nodes) and one for file data. When either an i-node or a file block is needed, a check is made to see if it can be satisfied out of the cache. If so, network traffic can be avoided.

While client caching helps performance enormously, it also introduces some nasty problems. Suppose that two clients are both caching the same file block and that one of them modifies it. When the other one reads the block, it gets the old (stale) value. The cache is not coherent. We saw the same problem with multiprocessors earlier. However, there it was solved by having the caches snoop on the bus to detect all writes and invalidate or update cache entries accordingly. With a file cache that is not possible, because a write to a file that results in a cache hit on one client does not generate any network traffic. Even if it did, snooping on the network is nearly impossible with current hardware.

Given the potential severity of this problem, the NFS implementation does several things to mitigate it. For one, associated with each cache block is a timer. When the timer expires, the entry is discarded. Normally, the timer is 3 sec for data blocks and 30 sec for directory blocks. Doing this reduces the risk somewhat. In addition, whenever a cached file is opened, a message is sent to the server to find out when the file was last modified. If the last modification occurred after the local copy was cached, the cache copy is discarded and the new copy fetched from the server. Finally, once every 30 sec a cache timer expires, and all the dirty (i.e., modified) blocks in the cache are sent to the server.

Still, NFS has been widely criticized for not implementing the proper UNIX semantics. A write to a file on one client may or may not be seen when another client reads the file, depending on the timing. Furthermore, when a file is created, it may not be visible to the outside world for as much as 30 sec. Similar problems exist as well.

From this example we see that although NFS provides a shared file system, because the resulting system is kind of a patched-up UNIX, the semantics of file access are not entirely well defined, and running a set of cooperating programs again may give different results, depending on the timing. Furthermore, the only issue NFS deals with is the file system. Other issues, such as process execution, are not addressed at all. Nevertheless, NFS is popular and widely used.

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