Introducing a distributed filesystem increases computer systems' dependence on the network. This dependency on file and data storage that people can only access over a network raises some interesting issues for laptop and mobile users who need access to their data even when they may not be directly connected to the network. This is known as disconnected operation, because the system needs to be able to function even when resources that it typically expects to use (such as user data) are not available in the standard fashion. Even a system like Windows provides integrated GUI and desktop features for marking files that you want to work with when you're not connected to the network, and for synchronizing those files when you reconnect.

The Coda and InterMezzo distributed filesystems that are currently available for Linux provide integrated support for offline operation, and work is also being done to provide this capability for NFS filesystems. Support for both the Coda and InterMezzo filesystems is already integrated into the mainline Linux kernel source - InterMezzo support has only been available in the kernel since version 2.4.5 or so, while Coda was integrated into the 2.4 kernel source from the beginning.

Coda is a distributed filesystem with its origin in AFS (the parent of OpenAFS), and has been under development at Carnegie Mellon University since 1987. InterMezzo is a relatively new distributed filesystem with a focus on high availability, flexible replication of directories, disconnected operation, and a persistent cache. InterMezzo was inspired by CMU's Coda, but is not based on the Coda source code. The initial creator of InterMezzo, Peter Braam, was the head of the Coda project at CMU for several years before moving on with InterMezzo and other advanced computing projects. Examples of using these filesystems will be provided in subsequent articles in this series.

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