Can I walk into a datacenter, find the mainframe's CD-ROM drive, and install Linux off my Red Hat CDs? Can I go to eBay, pick up a big honkin' S/390 for a few hundred bucks, load StarOffice, and have the fastest word processor in the world? Would that make me some sort of freak?

Part of the confusion stems from the alphabet soup of IBM's system lineup. If you want to buy a new system from IBM, your choices fall into one of four groups: The zSeries, which typically runs z/OS, are the mainframe systems. They're powered by 31-bit hardware. (Yes, that's 31.) Next down the line is the iSeries; these are the old AS/400s, with IBM's PowerPC chips inside. The same chips also are inside IBM's pSeries, which runs AIX, Big Blue's flavor of Unix. Finally, there's the xSeries -- all the Intel-based stuff.

In some way, shape or form, Linux runs on all of them. Those shapes and forms depend on IBM's open source code, and what Linux distributors like Red Hat and SuSE do with the code. There are two primary ways Linux runs on IBM's big iron. One way is natively. In other words, no other operating system or special software is required. It's the equivalent of wiping all traces of Windows off your computer, and replacing it with Linux or some other operating system. Linux can run natively on IBM's zSeries (the mainframes) and xSeries (Intel-based systems).

The other way is in a partition. For example, the native OS can be used to carve out a chunk of resources (like storage) and dedicate these resources to a separate instance of the native OS or another OS that IBM supports (like Linux). As a side note, the Datacenter Edition of Windows 2000 Server does this sort of partitioning, but it only allows you to run Windows in each partition. In most partitioned environments, the partitions don't interfere with each other. In essence, each partition is emulating a stand-alone computer running whatever OS was loaded into it. IBM has supported partitioning of its big iron into these sorts of virtual computers (IBM calls them "logical partitions" or "virtual machines") for what seems like forever. Whereas Linux can run natively on IBM's zSeries and xSeries, it can only run in partitions on the iSeries and pSeries.

One cool twist on the zSeries mainframes is that they can be partitioned physically as well as logically. This means that the mainframe can be carved into separate physical systems, each of which can run an OS (like Linux) in the native mode.

Why would IBM support this? It's pretty simple. Because Linux is a Unix variant, many enterprise applications can be ported to the open source OS. Once they're on the Linux code base, porting them to IBM's big-iron-specific Linux distributions is trivial. An example, according to SuSE vice president for Technology Partners Juergen Geck, is Software AG's database ADABAS/D. "Once Software AG ported the database to Linux," Geck says, "It took about a half a day to get it running on an IBM mainframe."

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