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Thomas Schenk
Senior Systems Administrator, Deja.com
01 May 2001

This article compares the differences and similarities between AIX and Linux. It also shows how they compliment each other in a network environment.

Linux vs. AIX
You may wonder why IBM would be interested in supporting Linux given they have their own brand of UNIX, AIX. One reason is Linux and AIX differ in their capabilities and, since they work well together, you can apply each to different types of problems. In this section, we will look at some of the differences between AIX and Linux as well as the similarities and how they compliment each other in a network environment.

Major differences
Most of the major differences between Linux and AIX stem from the fact that while Linux is well suited to running a interdepartmental server or even a small to medium sized Internet site, it lacks many of the features required to make it suitable for large scale systems. These issues are being addressed by Linux developers, but at the present, Linux is still best suited for less demanding tasks and really large, mission critical applications may be better served by AIX. Here are a few of the differences that AIX users will immediately notice when looking at Linux.

Logical volume manager
One of the first things most systems administrators will notice is that Linux does not include a logical volume manager or LVM. There is currently an LVM implementation in development, but it is not currently ready for heavy use. This lack means that space planning on Linux systems is much more difficult than under AIX since you are not able to extend filesystems once they start to run out of space. Although there is support in Linux for concatenating multiple physical disks into a single logical disk, you are not permitted to create multiple filesystems on the resulting disk as you are in AIX.

High availability
Another area where AIX outshines Linux is in high availability support. While AIX has long supported this feature and it is extremely well suited to applications requiring this type of support, Linux high availability options are currently very limited. Again, Linux developers are working on this issue, but if you need it today, your application may be better served by AIX.

Scalability
Scalability is where AIX has the greatest advantage over Linux. There is simply no comparison to be made between high-end AIX systems, like the RS/6000 which can support up to 24 processors and 64 GB of memory, and Linux systems (especially Linux systems on Intel). While Linux on UltraSparc and Alpha platforms outperform their Intel counterpart, even they fall short in this department. On the other hand, if you are looking for power workstations or small servers where 2 or 4 processors and 2 GB of memory are more than sufficient, then the Linux price, combined with the price advantage of Intel based systems, makes Linux a very attractive combination.

Similarities
As stated, Linux and AIX work well together in a network environment because of their similarities. One of the ways Linux and AIX can be used together is to use Linux as the operating system for workstations connected to AIX servers.

System V flavor
With the exception of the Slackware distribution, most Linux systems have a definite System V flavor, with the notable exception of the network software, which is based on BSD. For those familiar with AIX, this will sound quite familiar since AIX also started from a BSD base and later evolved into a more System V-based system. Signs of this include the default shells for Linux and AIX systems which are both Bourne shell compatible (ksh and bash) as opposed to the BSD C shell. AIX does diverge from System V systems and, thus, from Linux because it uses the BSD style of monolithic rc start-up scripts.

Standards compliance
Another area where AIX and Linux are similar is in the area of standards compliance. Both AIX and Linux are POSIX compatible systems. This means that software built to the POSIX specification should be relatively easy to port between the two systems.

Linux vs FreeBSD
Although Linux has captured most of the press recently, there are other open source implementations of UNIX available as well. One of the most popular is FreeBSD. FreeBSD is a project that evolved from the work done on the 386BSD project that was based on the Net-2 release of BSD. FreeBSD was later rewritten to remove all encumbered code and to move to a BSD 4.4 base. Touted by its advocates as more robust than Linux (especially in the area of networking), FreeBSD is an excellent alternative to Linux for those who prefer the BSD flavor of UNIX. In this section, we will look at some of the similarities and differences between Linux and FreeBSD.

Major differences
Since Linux was designed as a free implementation of the POSIX specification, based on System V Release 4, it differs in many ways from FreeBSD. Some of the major differences covered here are not related to this; but, they are more related to issues like SMP support, support by software vendors and the development model employed by the FreeBSD community.

SMP
One of the areas where Linux has an advantage on the FreeBSD community is in SMP, or multiprocessor support. Linux has supported SMP for about five years while FreeBSD has supported it for about two years. As a result, the Linux's support for SMP is considerably more mature than FreeBSD. Due to the open source nature of both systems, this will not be the case for long. FreeBSD developers have the opportunity to learn from the mistakes of the Linux developers.

Application support
Another area where the two communities differ is in support from commercial software vendors. In the past year, a significant number of software products were made available for the Linux platform with only a handful announced specifically for FreeBSD. However, the FreeBSD community quickly pointed out that the majority of software written for Linux will run on FreeBSD because of the Linux binary compatibility incorporated into their releases. Getting support from the software vendors for Linux binaries running under FreeBSD continues to be problematic.

Development model
If you have read this far, you already know that the Linux kernel is developed by a diverse group of programmers around the globe with project management duties delegated to Mr. Torvalds and a few key developers. In this model, Mr. Torvalds has the final say as to what goes into the kernel proper, but the development is basically open with new developers contributing constantly. The FreeBSD model is slightly more structured than this, with a central committee overseeing the additions to the FreeBSD system that includes not only the kernel but the base set of applications that make up a FreeBSD release as well. While this may seem a more restrictive model, BSD developers proclaim they prefer this model because, unlike Linux systems where everything but the kernel is put together by the distribution makers, FreeBSD systems are composed of the kernel. In addition, the basic utilities required to build an entire system are all from one distributor.

Similarities
Linux and FreeBSD are not totally different. Each relies on the large amount of open source software available on the Internet for their application base. In fact, there is a fair amount of code sharing between the two groups. There are other similarities.

Favored by ISPs
The proliferation of Internet service providers and the explosion of the World Wide Web can be attributed in part to the availability of Linux and FreeBSD. The ability of these operating systems to provide Internet services on relatively inexpensive hardware without requiring expensive software licenses makes both systems a favorite of ISPs all over the world. In addition to ISPs, these systems are also quite popular among Internet start-up companies.

Open source
The other major similarity between the two systems is that they are both open source projects. This means the source code for the systems is available to anyone with the inclination to download it from the Internet. There are many benefits to this, including the ability to react quickly to security flaws or other problems. A good example is the Pentium bug discovered a few years ago that caused system lockups when the processor was sent a specific instruction sequence, commonly referred to as the f00f bug. Both Linux and FreeBSD released patches to detect this defect and to prevent it from affecting their systems within hours of the f00f bug's announcement. That is quite impressive for a group of volunteers.

If you are interested in looking at FreeBSD, you can visit the FreeBSD Web site. As to which system is better, Linux or FreeBSD, I leave that to the advocates to argue. I run both.

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