Historically, it started in the middle 90s with the SP/2: this was a rack with several rack-mounted systems, some glue-logic to have a single management platform (called the "control workstation") and a proprietary highspeed network which connected all the systems ("nodes" in the SP/2 wording). There was also a middleware called "PSSP", which introduced further central manageability to this distributed system. It was IBMs take on so-called massively-parallel systems and it was initially designed for number-crunching. (for instance the "Deutscher Wetterdienst" [german weather forecast agency] was using a SP/2 with 64 frames full of thin-nodes - one of the fastest systems in the world in these times).

Maybe you remember the first computer beating an chess world champion: the "Deep Blue", which beated Garri Kasparov in 1997 - this was a two-frame SP/2 with 32 thin-nodes with additional specially developed chess processors added.

In fact it was often used as just another rack-mounted system with good central manageability and the massively parallel aspect of the system was often ignored. As well the wormhole-routing-based "High-Performance-Switch" was often (mis-)used as simple IP-interface. On these systems all the AIX versions from the beginning (well, probably not before 3.2.5, but nobody would have wnated to use any AIX prior to 3.2.5 anyway) could be run, but the current versions at this time were 4.x. 4.1.4 and 4.1.5 were relatively stable, 4.2 and 4.2.1 soon to be superseeded by 4.3 (4.2.x were, ahem, not the best effort IBM had undergone) and for a very long time there was 4.3.3 as the state of the art. 4.3.3 underwent 12 "maintenance levels" and this alone shows that for a very long time 4.3.3 was the best AIX one could use. It is - very rarely - seen even today on the occasional legacy system.

In about 2000 or 2001 IBM introduced the S/70, followed by the S/80 and similar systems. It took the idea of the massively-parallel system a little bit further and the management middleware PSSP "evolved" to "Cluster/1600", which was basically a makeover of PSSP with all the commandline tools removed and built into a GUI which everybody hated with a passion. While one could use the commandline tools to automate a lot of things (this was done a lot) with the GUI one could only use by hand. Many thanks for nothing, IBM. At the same time IBM issued the new AIX 5L which brought a massive change in the OS: the "L" was for "Linux affinity" and IBM started supporting Linux and tools from that part of the OS universe. There was, for instance, still the package manager from AIX (which is, btw. excellent - a lot better than the Linux counterparts), but you could install "rpm" additionally and work both package managers in parallel.

5.0 was buggy to now end and relatively fast replaced with 5.1, which was quite stable.

This was the state of matters when IBM in 2002/2003 (?) introduced the POWER4 processor and a completely new architecture: the "Regatta" (pSeries p6xx) was the successor of the high-end systems (S/80, S8a, ...) and the first system where a lot of resources were built into one box with the possibility to set aside portions of these resources (adapter cards, memory, processors, disks) and create several virtual systems - the LPAR was born. The first systems used AIX 5.0 and 5.1. There was also the HMC ("hardware management console), where the LPAR-profiles were being stored. This was a Linux-PC, heavily guarded by IBM middleware against a normal login as root by the systems owners, because IBM went "appliance-crazy" and the HMC was declared an "appliance", meaning its owner shouldn't be allowed to log in and use the system like any system else. If you search carefully you will find a lot of what one today would call "jailbreak how-tos" in the net explaining how to temporarily or permanently lift this completely unnecessary limitation. Still, this is so upon today, but IBMs measures against logging into your own HMC have quite evolved so that you need a lot of criminal energy today to break into the the system you bought.

In 2004 or 2005 IBM introduced, along with the next processor generation, the POWER5, the next generation of systems: p5xx. They were the absolutely fastest machines in the world and held all the OLTP-performance records, SPECint, etc.. The reason was the POWER5 processor, which was an absolutely phantastic thing. Also introduced was the "VIOS" (virtual I/O server). In the p6-series one had to have physical disks built into the system to boot the OS from. Data disks could be from SAN but the system had to be physical. The VIOS was (is - its still there) a special kind of LPAR, which gets LUNs via the SAN and presents them to other LPARs in the same hardware box as disks. This way an LPAR can boot from the SAN directly.

At the same time IBM presented a major makeover of their OS, AIX 5.2. This, for the first time, dropped the support of some outdated hardware while making use of the new hardware features of the the p5xx-systems. As AIX 5.2 was buggy to no end (oh my, this thing really sucked) AIX 5.3 came out in about 200

In about 2008 IBM introduced the POWER6 processor as successor of the POWER5. The systems were still called "p5xx", which is a bit confusing. Besides being faster, bigger, etc., the new systems supported "micro-partitioning", which means one could assign less than a complete processor to an LPAR. Another thing new was the "DLPAR" - dynamic LPAR. It was possible to move an LPAR from one hardware box to another, complete with profile, SAN disks and all WITHOUT ANY DOWNTIME! This was really remarkable, because no-one else in the industry had this. Users wouldn't even notice that their system was being moved from one box to another, because the performance loss was hardly noticeable and the process took only minutes.

Also new was the WPAR, the "workload partition". Basically this is like virtualization in the Linux world, only taking place in an LPAR: in the LPAR an OS ist started, then inside this several virtual machines are started with their resources taken from the LPAR. There is some management software to control how much every WPAR gets and what to do in cases of more WPARs competing for resources.

After a very long time of service AIX 5.3 was finally superseded by AIX 6.0, which quickly evolved into 6.1 in 2007. It supported the new features of the POWER6 hardware.

About a year ago IBM introduced the POWER7, which is again faster, bigger, etc. Only minimal improvements in functionality have surfaced, overall the very mature virtualization system has been straightened here, polished there - but no really big news.

The same is true for AIX 7.1, introduced at the end of 2010. Some more functionality with WPARs, more cores supported, etc.. Faster, bigger, better, but no really new functionality. After all, the system is very mature and one better doesn't fix things if they aren't broken.

I hope this helps.

bakunin