Linux Kernel Versions

Linux kernels come in two flavors: stable or development. Stable kernels are production-level releases suitable for widespread deployment. New stable kernel versions are released typically only to provide bug fixes or new drivers. Development kernels, on the other hand, undergo rapid change where (almost) anything goes. As developers experiment with new solutions, often-drastic changes to the kernel are made.

Linux kernels distinguish between stable and development kernels with a simple naming scheme (see Figure 1.2). Three numbers, each separated by a dot, represent Linux kernels. The first value is the major release, the second is the minor release, and the third is the revision. The minor release also determines whether the kernel is a stable or development kernel; an even number is stable, whereas an odd number is development. Thus, for example, the kernel version 2.6.0 designates a stable kernel. This kernel has a major version of two, has a minor version of six, and is revision zero. The first two values also describe the "kernel series"in this case, the 2.6 kernel series.

Development kernels have a series of phases. Initially, the kernel developers work on new features and chaos ensues. Over time, the kernel matures and eventually a feature freeze is declared. At that point, no new features can be submitted. Work on existing features, however, can continue. After the kernel is considered nearly stabilized, a code freeze is put into effect. When that occurs, only bug fixes are accepted. Shortly thereafter (one hopes), the kernel is released as the first version of a new stable series. For example, the development series 1.3 stabilized into 2.0 and 2.5 stabilized into 2.6.

Everything I just told you is a lie Well, not exactly. Technically speaking, the previous description of the kernel development process is true. Indeed, historically the process has proceeded exactly as described. In the summer of 2004, however, at the annual invite-only Linux Kernel Developers Summit, a decision was made to prolong the development of the 2.6 kernel without introducing a 2.7 development series in the near future. The decision was made because the 2.6 kernel is well received, it is generally stable, and no large intrusive features are on the horizon. Additionally, perhaps most importantly, the current 2.6 maintainer system that exists between Linus Torvalds and Andrew Morton is working out exceedingly well. The kernel developers believe that this process can continue in such a way that the 2.6 kernel series both remains stable and receives new features. Only time will tell, but so far, the results look good.

This book is based on the 2.6 stable kernel series.