First and foremost, enough of you responded to thoroughly convince me that Linux on the mainframe isn't just an interesting idea. It's something that the mainframe experts at many companies are not only enthusiastic about, but are seriously considering for all sorts of applications and server consolidation opportunities.
Beyond that, many of you seized the opportunity to make sure that some of the more granular, salient details of running Linux on IBM's big iron didn't elude me.
IBM systems programmer Phil Hubert wanted to make sure that my comments about IBM's 31-bit architecture didn't mislead readers about the capabilities of IBM's zSeries. According to Hubert, "IBM's zServer and zOS were indeed 64-bit and were replacements for the S/390 and OS/390 products."
BMC lead product developer William Blair provided a detailed history of IBM's mainframes. According to Blair, "The old IBM mainframes were actually 32-bit hardware (and were so from the first System/360 in 1965). However, until 1981, you could only address 24 or 25 bits of central storage. After 1981, starting with the '3081' series of System/370 processors, you could address a maximum of 31 bits of central storage. However, all these machines were '32-bit processors.' "
Apparently, that doesn't mean the zSeries makes all the old 24 and 31-bit stuff obsolete. Phil Payne of research outfit Isham Research added that even though IBM's zSeries products are 64-bit, that doesn't prevent them from running legacy subsystems and applications that rely on the older 31- and 24-bit addressing modes.
Jim Elliot, a Linux advocate from IBM Canada, chimed in to confirm Hubert, Blair, and Payne's assertion that the S/390's 31-bit and 24-bit modes actually use a 32-bit architecture. But depending on the sort of partitioning you do with the zSeries says Elliot, the 64-, 31- and 24-bit modes can all run simultaneously on one system.
Why does this matter? BMC's Blair points out the old "if it ain't broke don't fix it mentality" that conservative mainframe shops often stick to. Says Blair, "What's important, and remains important, is that the original System/360 machines were 32-bit architecture, and the same programs that ran on OS/360 in 1965 can still be run today, unchanged, on a zSeries processor under z/OS. In some cases, such programs are still running."
What does any of this have to do with Linux? Well, for starters--according to Blair--the zSeries' 64-bit nature makes it possible to allocate more than 2GB of central storage to any of a zServer's logical hardware partitions (LPARs). I started to think about how that sort of addressability might come in handy when attempting to consolidate lots of servers and applications onto one "box," thereby simplifying systems management and lowering total cost of ownership.
Server partitioning
According to Scott Courtney, a senior engineer at large scale systems and network integrator Sinenomine, you can use LPARs to carve a zSeries mainframe into fifteen physical partitions or resource pools, each of which can be further divided into multiple virtual Linux machines using z/VM. Doing so, says Courtney, provides a host of further advantages. "For starters, you are not limited to 15 Linux machines. My company has customers that are running hundreds of Linux virtual machines on a single mainframe, within a single LPAR, while legacy mainframe batch systems continue to run on other LPARs. In a test-lab situation, one of our engineers has successfully booted over 97,000 Linux virtual machines simultaneously on a single mainframe, with each of these Linux instances running a Web server application under simulated load. We don't recommend trying to consolidate 97,000 servers onto one mainframe--this was just a lab test--but several hundred is no problem at all, depending on the nature of the application(s)."
Talk about server consolidation!
Courtney says this approach is inherently better than running Linux on a mainframe natively: "While certainly possible, [it] is very rarely practical from an economic standpoint. What you get is a fast PC that costs over $500,000--not a very good deal. Running Linux virtual machines within LPARs makes more sense, because you can divide functionality among various Linux instances in order to make things more manageable and to consolidate other servers onto the mainframe."
At least one of the folks at IBM Global Service agrees. According to IGS IT architect Stephen Rifkin, "Running Linux in an LPAR in Z hardware is less efficient than running multiple Linux guests under VM hypervisor, which is the preferred approach. We run several dozen or more instances of Linux as logically distinct guests of VM, and none of the instances share logical resources, only physical resources. If you want to run Linux based routers and firewalls inside your mainframe this is the way to go."
Oh, and about the pSeries
Several readers, including IBM's Eivind Trondsen, corrected me on the issue of running Linux natively on the pSeries machines (which I said couldn't be done). According to Trondsen, "You can run Linux natively on the smaller pSeries boxes."
Finally, several readers mistook the rhetorical nature of my question about buying a mainframe on eBay for US$200 and running StarOffice on it as being serious. Some found it to be ignorant and insulting, but one pointed out that I should be careful what I wish for. Dave Charlton thought that I might be surprised to see what his brother picked up on eBay last month for a cool two Bennies.
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