Книга: Distributed operating systems

10.7.3. DFS Components in the Client Kernel

10.7.3. DFS Components in the Client Kernel

The main addition to the kernel of each client machine in DCE is the DFS cache manager. The goal of the cache manager is to improve file system performance by caching parts of files in memory or on disk, while at the same time maintaining true UNIX single-system file semantics. To make it clear what the nature of the problem is, let us briefly look at UNIX semantics and why this is an issue.

In UNIX (and all uniprocessor operating systems, for that matter), when one process writes on a file, and then signals a second process to read the file, the value read must be the value just written. Getting any other value violates the semantics of the file system.

This semantic model is achieved by having a single buffer cache inside the operating system. When the first process writes on the file, the modified block goes into the cache. If the cache fills up, the block may be written back to disk. When the second process tries to read the modified block, the operating system first searches the cache. Failing to find it there, it tries the disk. Because there is only one cache, under all conditions, the correct block is returned.

Now consider how caching works in NFS. Several machines may have the same file open at the same time. Suppose that process 1 reads part of a file and caches it. Later, process 2 writes that part of the file. The write does not affect the cache on the machine where process 1 is running. If process 1 now rereads that part of the file, it will get an obsolete value, thus violating the UNIX semantics. This is the problem that DFS was designed to solve.

Actually, the problem is even worse than just described, because directories can also be cached. It is possible for one process to read a directory and delete a file from it. Nevertheless, another process on a different machine may subsequently read its previously cached copy of the directory and still see the now-deleted file. While NFS tries to minimize this problem by rechecking for validity frequently, errors can still occur.

DFS solves this problem using tokens. To perform any file operation, a client makes a request to the cache manager, which first checks to see if it has the necessary token and data. If it has both the token and the data, the operation may proceed immediately, without contacting the file server. If the token is not present, the cache manager does an RPC with the file server asking for it (and for the data). Once it has acquired the token, it may perform the operation.

Tokens exist for opening files, reading and writing files, locking files, and reading and writing file status information (e.g., the owner). Files can be opened for reading, writing, both, executing, or exclusive access. Open and status tokens apply to the entire file. Read, write, and lock tokens, in contrast, refer only to some specific byte range. Tokens are granted by the token manager in Fig. 10-31(b).

Figure 10-32 gives an example of token usage. In message 1, client 1 asks for a token to open some file for reading, and also asks for a token (and the data) for the first chunk of the file. In message 2, the server grants both tokens and provides the data. At this point, client 1 may cache the data it has just received and read it as often as it likes. The normal transfer unit in DFS is 64K bytes.

A little later, client 2 asks for a token to open the same file for reading and writing, and also asks for the initial 64K of data to selectively overwrite part of it. The server cannot just grant these tokens, since it no longer has them. It must send message 4 to client 1 asking for them back.


Fig. 10-32. An example of token revocation.

As soon as is reasonably convenient, client 1 must return the revoked tokens to the server. After it gets the tokens back, the server gives them to client 2, which can now read and write the data to its hearts' content without notifying the server. If client 1 asks for the tokens back, the server will instruction client 2 to return the tokens along with the updated data, so these data can be sent to client 1. In this way, single-system file semantics are maintained.

To maximize performance, the server understands about token compatibility. It will not simultaneously issue two write tokens for the same data, but it will issue two read tokens for the same data, provided that both clients have the file open only for reading.

Tokens are not valid forever. Each token has an expiration time, usually two minutes. If a machine crashes and is unable (or unwilling) to return its tokens when asks, the file server can just wait two minutes and then act like they have been returned.

On the whole, caching is transparent to user processes. However, calls are available for certain cache management functions, such as prefetching a file before it is used, flushing the cache, disk quota management, and so on. The average user does not need these, however.

Two differences between DFS and its predecessor, AFS, are worth mentioning. In AFS, entire files were transferred, instead of 64K chunks. This strategy made a local disk essential, since files were often too large to cache in memory. With a 64K transfer unit, local disks are no longer required.

A second difference is that in AFS part of the file system code was in the kernel and part was in user space. Unfortunately, the performance left much to be desired, so in DFS it is all in the kernel.

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