DFS Components

DFS consists of several components. This topic briefly describes each of these components, discussing the software that runs on DCE client machines (the cache manager), then the software that runs on DCE file server machines (the file exporter, token manager, and DCE Local File System), and finally the administrative server processes, which typically run on DFS File Server machines (the fileset server, basic overseer server, replication server, update server, fileset location server, and backup server). It also briefly describes the administrative tools used to monitor DFS use and activity (Scout and the dfstrace utility), and it describes the DFS/NFS secure gateway, which provides authenticated access to DFS from NFS clients.

Cache Manager

The cache manager is the client side of DFS. The cache manager runs on any machine that is acting as a DFS client. It takes a user's file system request and looks in a local cache to see if a copy of the data is already on the local system. If it does not find the data in the local cache, the cache manager sends a request for the data to the file server machine and caches the data locally, either on disk or in memory.

Because files are cached on the client, a local copy of a cached file can subsequently be accessed instead of the remote copy on the file server machine. As a result, network traffic to the file server machine, as well as file server machine load, is much lighter than if the client had to go to the server each time it needed to access a file.

File Exporter

The file exporter is the server side of DFS. The file exporter runs on a DFS file server machine, where it handles requests from clients for the files that it manages. The file exporter receives an RPC call and accesses its own local file system, which can be the DCE Local File System (LFS) or another file system such as a UNIX File System (UFS), to service the request. Using the token manager, it handles the synchronization of different clients concurrently accessing the same file and returns the requested information to the client.

Token Manager

The token manager runs on a file server machine to synchronize access to files by multiple clients. It does this by issuing tokens, which represent the ability to perform operations. The tokens that a token manager issues to DFS clients carry various access rights, usually read or write. There are four different kinds of tokens: data tokens for access to file and directory data, status tokens for access to file and directory status, lock tokens for locking a portion of a file, and open tokens for opening a file.

The token manager on the server side cooperates with the token management layer in the cache manager (on the client side) to manage tokens. If a client requests an operation that conflicts with a token that another client holds, the token manager must revoke the existing token and grant a new token before the requested operation can proceed.

DCE Local File System

The DCE Local File System (DCE LFS) is the physical file system provided with DCE. It manages the storage of files on a disk. The scope of DCE LFS is a single computer. LFS is analogous to a UNIX file system. However, LFS is more powerful than most local UNIX file systems - it includes features that result in greater capabilities than a distributed file service based on a traditional UNIX file system. These capabilities include the ability to use more flexible data protection in the form of DCE access control lists (ACLs); the ability to replicate, back up, and even move different parts of the file system without interruption to service; and the use of logging for fast recovery after a crash (in contrast to UNIX file systems, which must execute the time-consuming fsck command). DCE LFS also includes support for DCE cells; for example, the owner of a file or the name in an entry on an access control list can be a name from a foreign cell.

A UNIX File System (UFS) can be used as a file server machine's physical file system as an alternative or complement to DCE LFS. DFS can export a UFS, issue synchronization tokens for files in a UFS, and perform fileset operations such as dump and restore on a UFS. However, there is only one fileset per UFS partition, which results in large filesets, and unlike DCE LFS filesets, UFS filesets cannot be replicated or moved. Although UFS systems are supported in DFS, a file server machine that uses DCE LFS has more functionality than a file server machine that uses only UFS.

Fileset Server

The fileset server allows administrators to create, delete, move, and perform other operations on filesets. For example, the fileset server enables an administrator to move a fileset from one file server machine to another for load balancing. (If DCE LFS is not being used as the physical file system, an entire partition is treated as a single fileset; in this case, some fileset operations may not be supported.)

Basic Overseer Server

The basic overseer server, or BOS server, monitors the DFS processes that run on a server and restarts them when needed. The BOS server maintains information about the processes and responds to administrative requests for that information.

Replication Server

The replication server is an administrative server that handles replication of filesets. For example, an administrator can create read-only copies of a fileset on multiple file server machines. The replication server updates the replicas either manually, at the request of an administrator, or automatically, as data in the fileset changes. With replication, even if a file server machine that houses one copy of a fileset goes down, another copy of the fileset is still available on another file server machine.

Update Server

The update server provides the ability to distribute binary files or administrative information to machines configured as DFS servers. The update server consists of the upclient and upserver processes. The upclient software runs on a machine that needs to receive new versions of the binary files or administrative information. The upserver software runs on a master machine and on request propagates any changes to binaries or administrative information to the machines running the upclient software.

Fileset Location Server

The fileset location server, or FL server, provides a replicated directory service that keeps track of the site (file server machine and aggregate) at which each fileset resides. The FL server provides a lookup service analogous to the service CDS provides, with the exception that the FL server is specialized for DFS. It provides fileset location transparency: Users can access a fileset simply by knowing its name; they do not need to know the fileset's location. As a result, a fileset can be moved without users and applications being aware of the move. DFS automatically updates the fileset's location in the fileset location database (FLDB).

Backup Server

The backup server is a facility for backing up data on file server machines. The backup server maintains backup records in the replicated backup database. It maintains a schedule for the backing up of file system data, and it has the ability to perform both full and incremental dumps. The unit of backup is the fileset.


The Scout administrative tool collects and displays information about the file exporters running on file server machines, enabling a system administrator to monitor the use of DFS.

The dfstrace Utility

The dfstrace utility allows sophisticated administrators and system developers to trace DFS processes that run in either the user-space or the kernel. The utility consists of a suite of commands that provide low-level diagnostic and debugging information.

DFS/NFS Secure Gateway

The DFS/NFS Secure Gateway provides authenticated access to DFS from NFS clients. Users who have DCE accounts can authenticate to DCE via a DFS client configured as a gateway server and access DFS data according to their DCE identities. Administrators can give users the ability to authenticate to DCE from NFS clients, or administrators can reserve the ability to grant authenticated access from a gateway server only. Some DFS components run in the host machine's kernel. These are the cache manager and token management layer on DFS client machines; and the file exporter, token manager, and DCE local file system on file server machines.