| Explaining BSD | ||
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So what is really the difference between, say, Debian Linux and FreeBSD? For the average user, the difference is surprisingly small: Both are UNIX® like operating systems. Both are developed by non-commercial projects (this does not apply to many other Linux distributions, of course). In the following section, we will look at BSD and compare it to Linux. The description applies most closely to FreeBSD, which accounts for an estimated 80% of the BSD installations, but the differences from NetBSD, OpenBSD and DragonFlyBSD are small.
No one person or corporation owns BSD. It is created and distributed by a community of highly technical and committed contributors all over the world. Some of the components of BSD are Open Source projects in their own right and managed by different project maintainers.
The BSD kernels are developed and updated following the Open Source development model. Each project maintains a publicly accessible source tree under the Concurrent Versions System (CVS), which contains all source files for the project, including documentation and other incidental files. CVS allows users to “check out” (in other words, to extract a copy of) any desired version of the system.
A large number of developers worldwide contribute to improvements to BSD. They are divided into three kinds:
Contributors write code or documentation. They are not permitted to commit (add code) directly to the source tree. In order for their code to be included in the system, it must be reviewed and checked in by a registered developer, known as a committer.
Committers are developers with write access to the source tree. In order to become a committer, an individual must show ability in the area in which he is active.
It is at the individual committer's discretion whether he should obtain authority before committing changes to the source tree. In general, an experienced committer may make changes which are obviously correct without obtaining consensus. For example, a documentation project committer may correct typographical or grammatical errors without review. On the other hand, developers making far-reaching or complicated changes are expected to submit their changes for review before committing them. In extreme cases, a core team member with a function such as Principal Architect may order that changes be removed from the tree, a process known as backing out. All committers receive mail describing each individual commit, so it is not possible to commit secretly.
The Core team. FreeBSD and NetBSD each have a core team which manages the project. The core teams developed in the course of the projects, and their role is not always well-defined. It is not necessary to be a developer in order to be a core team member, though it is normal. The rules for the core team vary from one project to the other, but in general they have more say in the direction of the project than non-core team members have.
This arrangement differs from Linux in a number of ways:
No one person controls the content of the system. In practice, this difference is overrated, since the Principal Architect can require that code be backed out, and even in the Linux project several people are permitted to make changes.
On the other hand, there is a central repository, a single place where you can find the entire operating system sources, including all older versions.
BSD projects maintain the entire “Operating System”, not only the kernel. This distinction is only marginally useful: neither BSD nor Linux is useful without applications. The applications used under BSD are frequently the same as the applications used under Linux.
As a result of the formalized maintenance of a single CVS source tree, BSD development is clear, and it is possible to access any version of the system by release number or by date. CVS also allows incremental updates to the system: for example, the FreeBSD repository is updated about 100 times a day. Most of these changes are small.
FreeBSD, NetBSD and OpenBSD provide the system in three different “releases”. As with Linux, releases are assigned a number such as 1.4.1 or 3.5. In addition, the version number has a suffix indicating its purpose:
The development version of the system is called CURRENT. FreeBSD assigns a number to CURRENT, for example FreeBSD 5.0-CURRENT. NetBSD uses a slightly different naming scheme and appends a single-letter suffix which indicates changes in the internal interfaces, for example NetBSD 1.4.3G. OpenBSD does not assign a number (“OpenBSD-current”). All new development on the system goes into this branch.
At regular intervals, between two and four times a year, the projects bring out a RELEASE version of the system, which is available on CD-ROM and for free download from FTP sites, for example OpenBSD 2.6-RELEASE or NetBSD 1.4-RELEASE. The RELEASE version is intended for end users and is the normal version of the system. NetBSD also provides patch releases with a third digit, for example NetBSD 1.4.2.
As bugs are found in a RELEASE version, they are fixed, and the fixes are added to the CVS tree. In FreeBSD, the resultant version is called the STABLE version, while in NetBSD and OpenBSD it continues to be called the RELEASE version. Smaller new features can also be added to this branch after a period of test in the CURRENT branch.
By contrast, Linux maintains two separate code trees: the stable version and the development version. Stable versions have an even minor version number, such as 2.0, 2.2 or 2.4. Development versions have an odd minor version number, such as 2.1, 2.3 or 2.5. In each case, the number is followed by a further number designating the exact release. In addition, each vendor adds their own userland programs and utilities, so the name of the distribution is also important. Each distribution vendor also assigns version numbers to the distribution, so a complete description might be something like “TurboLinux 6.0 with kernel 2.2.14”
In contrast to the numerous Linux distributions, there are only four major open source BSDs. Each BSD project maintains its own source tree and its own kernel. In practice, though, there appear to be fewer divergences between the userland code of the projects than there is in Linux.
It is difficult to categorize the goals of each project: the differences are very subjective. Basically,
FreeBSD aims for high performance and ease of use by end users, and is a favourite of web content providers. It runs on a number of platforms, including i386™ based systems (“PCs”), systems based on the AMD 64-bit processors, UltraSPARC® based systems, systems based on Compaq's Alpha processors and systems based around the NEC PC-98 specification. The FreeBSD project has significantly more users than the other projects.
NetBSD aims for maximum portability: “of course it runs NetBSD”. It runs on machines from palmtops to large servers, and has even been used on NASA space missions. It is a particularly good choice for running on old non-Intel® hardware.
OpenBSD aims for security and code purity: it uses a combination of the open source concept and rigorous code reviews to create a system which is demonstrably correct, making it the choice of security-conscious organizations such as banks, stock exchanges and US Government departments. Like NetBSD, it runs on a number of platforms.
DragonFlyBSD aims for high performance and scalability under everything from a single-node UP system to a massively clustered system. DragonFlyBSD has several long-range technical goals, but focus lies on providing a SMP-capable infrastructure that is easy to understand, maintain and develop for.
There are also two additional BSD UNIX operating systems which are not open source, BSD/OS and Apple's Mac OS® X:
BSD/OS was the oldest of the 4.4BSD derivatives. It was not open source, though source code licenses were available at relatively low cost. It resembled FreeBSD in many ways. Two years after the acquisition of BSDi by Wind River Systems, BSD/OS failed to survive as an independent product. Support and source code may still be available from Wind River, but all new development is focused on the VxWorks embedded operating system.
Mac OS X is the latest version of the operating system for Apple Computer Inc.'s Macintosh® line. The BSD core of this operating system, Darwin, is available as a fully functional open source operating system for x86 and PPC computers. The Aqua/Quartz graphics system and many other proprietary aspects of Mac OS X remain closed-source, however. Several Darwin developers are also FreeBSD committers, and vice-versa.
Linux is available under the GNU General Public License (GPL), which is designed to eliminate closed source software. In particular, any derivative work of a product released under the GPL must also be supplied with source code if requested. By contrast, the BSD license is less restrictive: binary-only distributions are allowed. This is particularly attractive for embedded applications.
Since fewer applications are available for BSD than Linux, the BSD developers created a Linux compatibility package, which allows Linux programs to run under BSD. The package includes both kernel modifications, in order to correctly perform Linux system calls, and Linux compatibility files such as the C library. There is no noticeable difference in execution speed between a Linux application running on a Linux machine and a Linux application running on a BSD machine of the same speed.
The “all from one supplier” nature of BSD means that upgrades are much easier to handle than is frequently the case with Linux. BSD handles library version upgrades by providing compatibility modules for earlier library versions, so it is possible to run binaries which are several years old with no problems.
What does this all mean in practice? Who should use BSD, who should use Linux?
This is a very difficult question to answer. Here are some guidelines:
“If it ain't broke, don't fix it”: If you already use an open source operating system, and you are happy with it, there is probably no good reason to change.
BSD systems, in particular FreeBSD, can have notably higher performance than Linux. But this is not across the board. In many cases, there is little or no difference in performance. In some cases, Linux may perform better than FreeBSD.
In general, BSD systems have a better reputation for reliability, mainly as a result of the more mature code base.
BSD projects have a better reputation for the quality and completeness of their documentation. The various documentation projects aim to provide actively updated documentation, in many languages, and covering all aspects of the system.
The BSD license may be more attractive than the GPL.
BSD can execute most Linux binaries, while Linux can not execute BSD binaries. Many BSD implementations can also execute binaries from other UNIX like systems. As a result, BSD may present an easier migration route from other systems than Linux would.
BSDi / FreeBSD Mall, Inc. have been providing support contracts for FreeBSD for nearly a decade.
In addition, each of the projects has a list of consultants for hire: FreeBSD, NetBSD, and OpenBSD.
This, and other documents, can be downloaded from ftp://ftp.FreeBSD.org/pub/FreeBSD/doc/.
For questions about FreeBSD, read the documentation before contacting <questions@FreeBSD.org>.
For questions about this documentation, e-mail <doc@FreeBSD.org>.