My thoughts on the advantages/disadvantages of Linux on a IBM zSeries Mainframe (zLinux)

In response to someone commenting on a post, "Why would anyone want to use z Linux?", I decided to share my thoughts on some of the advantages and disadvantages of running zLinux in your infrastructure: First, the advantages:

1. Insanely fast RISC processors

The z196 (which was the latest model I worked with) had "CPU Engines" up to 5.2 GHz. They also 3 layers of CPU cache in large quantities (24MB per core). I'm not sure what the hottest x86 processor is now adays but the Nehalem only has 12MB/chip. http://en.wikipedia.org/wiki/Nehalem_(microarchitecture) http://en.wikipedia.org/wiki/IBM_z196_(microprocessor)

2. Pure hardware virtualization

The hypervisor doesn't emulate anything -- you're guests are talking directly do the devices. It is true hardware virtualization and all of the guests are shared among the CPU engines in whatever way you want. The underlying hypervisor is solid (possibly because it's like 50 years old).

3. Licensing benefits

If you have software that is licensed by the core, then one x86 server might require you to license it for 16 cores, even if you do fancy stuff like pinning to a CPU on ESX, etc. With a mainframe you can run many hundreds of "guests" on 16 engines and run tons of instances of your software.

4. You may already have a z mainframe in your datacenter running some z/OS workloads

Why not add a few linux engines (which are already in the mainframe...you just have to pay IBM and turn them on) extend it to also run linux!

5. There are some workloads that are the "sweet spot" of running on a mainframe

I think many zLinux shops choose to run Oracle DB's because the workload fits the z nicely and they can take advantage of licensing (see #3). The problem there is making sure you're exploiting these in the correct way. Not every workload makes sense on a mainframe and you should have proper analytics to justify this.

6. I feel like the open source community is somewhat in line with supporting the s/390 architecture

Last I knew Fedora released s/390 builds at the same time as the other architectures. A colleague had it Fedora 15 up and running if I recall. I think there are CentOS builds too.

Some of the disadvantages

1. It is highly specialized

These are COMPLETELY different operating systems than anything mainstream e.g. unix/linux/windows/bsd/anything you can think of. You don't have bash. You might have some really old versions of scripting langauges (e.g. early python and perl) that somebody compiled to run. You'll probably need to know ReXX.

2. Finding people that know what they're doing is getting more difficult

As the older generation that know these systems are retiring. This is why Nationwide is doing z University to fill those gaps. http://bit.ly/ZMVVjN

3. They aren't cheap

They have smaller versions but still those aren't cheap

4. Finding people to run them aren't cheap

I was always told if you're in the zLinux world you write your own paychecks

5. Software support is nearly non-existent or not possible in many cases

Redhat and SuSE will sell you an operating system and you'll pay to support it. But unless the vendor of your favorite software specifically writes and compiles their software for zLinux it likely wont work. And even if you are able to get it to work, they can't/won't support you because they probably don't have a mainframe sitting around test on nor the platform expertise. You can be sure IBM either already has some type of solution for whatever you're trying to do, or they'll build it for you and charge you out the ass.

These observartions are based purely on my experience in working in an org running zLinux. YMMV :)

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