Thompson says Plan 9 "the best operating system out there" but will "never make it, ever, ever."

Plan 9: Design and Assessment

Ken Thompson said he loved Plan 9, describing it as clean, precise, attractive, and capable. However, he believed it would not become widely successful because it lacked applications users expected—especially a web browser as internet use took off.

Distributed Design

Plan 9 was designed to connect multiple machines so they could be used as though they were nearby or far away. It handled a user’s terminal, keyboard, and mouse through interfaces that could be multiplexed and directed to different CPU servers.

File-Based Interfaces

The system used a multiplexing protocol built around basic operations such as open, close, read, and write. This protocol could present a hierarchy of files, including interfaces for the terminal, mouse, and windows.

A window system could split a terminal into separate sessions, potentially connected to different servers.

Versioned, Economical Storage

Plan 9’s cached file system used WORM storage (“write once, read many”) for immutable data, while writable portions were kept on disk. Backups created versions of the file system and retained earlier ones, allowing users to recover from large changes without manually copying many files.

Thompson valued this feature because his programming work often involved broad changes that he sometimes wanted to undo.

Source and Context

The clip is Thompson’s retrospective explanation and evaluation of Plan 9, with Rob Pike credited as its principal designer. It does not present a formal review, guide, or tutorial.