UNIX Filesystems: Evolution, Design, and Implementation

5th to 7th Edition Internals

From the mid 1980s onwards, there have been a number of changes in the UNIX kernel that resulted in the mainstream kernels diverging in their implementation. For the first fifteen years of UNIX development, there wasn't a huge difference in the way many kernel subsystems were implemented, and therefore understanding the principles behind these earlier UNIX versions will help readers understand how the newer kernels have changed.

The earliest documented version of UNIX was 6th Edition, which can be seen in John Lions' book Lions' Commentary on UNIX 6th Edition — with source code [LION96]. It is now also possible to download free versions of UNIX from 5th Edition onwards. The kernel source base is very small by today's standards. With less than 8,000 lines of code for the whole kernel, it is easily possible to gain an excellent understanding of how the kernel worked. Even the small amounts of assembler code do not need significant study to determine their operation.

This chapter concentrates on kernel principles from a filesystem perspective. Before describing the newer UNIX implementations, it is first necessary to explain some fundamental UNIX concepts. Much of the description here centers around the period covering 5th to 7th Edition UNIX, which generally covers the first ten years of UNIX development. Note that the goal here is to avoid swamping the reader with details; therefore, little knowledge of UNIX kernel internals is required in order to read ...