Understanding Plan 9: The Distributed Research OS

0.1 What is Plan 9?

Plan 9 is a distributed research operating system developed at Bell Labs' Computing Sciences Research Center by the creators of UNIX. Emerging in the late 1980s, it extended UNIX principles into the networking and graphics era. Rob Pike described it as "an argument" for simplicity and clarity in system design.

Core Principles

Two fundamental ideas define Plan 9:

1. Private Namespaces: Each process constructs its own hierarchical file system view
2. File Interfaces: All system resources (devices, network sockets, etc.) appear as file systems

This design enables a distributed environment where separate machines function as:

• Terminals (user workstations)
• CPU servers (computation resources)
• File servers (centralized storage)

All resources communicate via a unified file system interface, allowing operations like controlling devices through ASCII strings instead of specialized APIs.

0.1.1 Plan 9 ≠ UNIX

While sharing UNIX concepts, Plan 9 is architecturally distinct:

• New kernel and subsystems designed without backward-compatibility constraints
• Clean-slate implementation of ideas prototyped in Research UNIX
• Namespace customization per process (unlike UNIX's global filesystem view)

As Dennis Ritchie noted: "The set of files an individual program can see depends on that program itself... adjustable per program."

0.1.1.1 Plan 9 ≠ plan9port

plan9port is a partial userspace adaptation for UNIX-like systems:

• Provides Plan 9 tools (Acme/Sam editors) on Linux/macOS
• Doesn't replicate the complete Plan 9 experience
• Lacks the kernel-level integration of actual Plan 9

0.1.1.2 Plan 9 ≠ Inferno

Inferno is a separate distributed OS:

• Shares Plan 9's 9P protocol (renamed Styx)
• Focuses on portable virtual machines
• Developed concurrently but maintained independently

0.1.2 Research Focus

Plan 9 was never a commercial product. As Steven Bellovin clarified in 1988:

"Plan 9 is not a product, and is not intended to be. It is research - an experimental investigation into a different way of computing."

The system prioritized:

• Distributed computing paradigms
• Unified resource access
• Clean-slate OS design

0.2 Why Plan 9?

Technical Advantages

Russ Cox highlights Plan 9's consistency:

"Once you've settled in, there are very few surprises here."

Key innovations include:

• Network transparency: Import remote resources (e.g., /dev/audio from another machine)
• User-level file servers: Implement filesystems without kernel modifications
• Per-process namespaces: Custom environments for applications

Practical Limitations

Despite technical merits, adoption barriers included:

• Limited hardware driver support
• Minimal commercial application ecosystem
• Browser gap (inability to run contemporary web browsers)

As Rob Pike noted: "I switched to Linux but found it very buggy... however, it was just too inconvenient to live on a machine without a web browser."

Legacy and Derivatives

Original Plan 9 development ceased when core developers left Bell Labs. Active forks include:

• 9front: Modernized fork with new drivers/filesystems
• 9legacy: Maintenance branch of original Labs code
• Harvey: Plan 9 adaptation for GCC/Clang toolchains

Further Reading

• Academic Papers
• Plan 9 Manual
• Designing Plan 9 (original design document)