Overview of Cache Manager Customization

An AFS client machine's kernel includes a set of modifications, commonly referred to as the Cache Manager, that enable access to AFS files and directories and communications with AFS server processes. It is common to speak of the Cache Manager as a process or program, and in regular usage it appears to function like one. When configuring it, though, it is helpful to keep in mind that this usage is not strictly accurate.

The Cache Manager mainly fetches files on behalf of application programs running on the machine. When an application requests an AFS file, the Cache Manager contacts the Volume Location (VL) Server to obtain a list of the file server machines that house the volume containing the file. The Cache Manager then translates the application program's system call requests into remote procedure calls (RPCs) to the File Server running on the appropriate machine. When the File Server delivers the file, the Cache Manager stores it in a local cache before delivering it to the application program.

The File Server delivers a data structure called a callback along with the file. (To be precise, it delivers a callback for each file fetched from a read/write volume, and a single callback for all data fetched from a read-only volume.) A valid callback indicates that the Cache Manager's cached copy of a file matches the central copy maintained by the File Server. If an application on another AFS client machine changes the central copy, the File Server breaks the callback, and the Cache Manager must retrieve the new version when an application program on its machine next requests data from the file. As long as the callback is unbroken, however, the Cache Manager can continue to provide the cached version of the file to applications on its machine, which eliminates unnecessary network traffic.

The indicated sections of this chapter explain how to configure and customize the following Cache Manager features. All but the first (choosing disk or memory cache) are optional, because AFS sets suitable defaults for them.

You must make all configuration changes on the client machine itself (at the console or over a direct connection such as a telnet connection). You cannot configure the Cache Manager remotely. You must be logged in as the local superuser root to issue some commands, whereas others require no privilege. All files mentioned in this chapter must actually reside on the local disk of each AFS client machine (they cannot, for example, be symbolic links to files in AFS).