Starting with the IBM z9 mainframe, and even more so with the z10 it introduced this year, IBM has promoted big iron as a server consolidation savior – in particular, for x86 Linux server consolidation.
IBM has hardly been shy about supporting a Linux-only mainframe in which hundreds or even thousands of virtual Linux servers run on z/VM, along with a bunch of Integrated Facility for Linux, or IFL processors -- but without the presence of z/OS, IBM's landmark mainframe operating system.
But, according to IBM employees and analysts, the reasons for a purely Linux mainframe have more to do with politics and logistics than with technology.
Reasons for deploying a Linux-only mainframe
Jim Porell, the chief architect of IBM System z software, pointed to a Japanese bank as an example. Though he wouldn't give the name of the bank, he said it was running its entire operations on IFLs alone. Why? Because logistically it made sense.
Porell said this bank "skipped the client server revolution and decided that it was appropriate to add Linux (on the mainframe) to their business at this time."
In another widely publicized case, Nationwide Insurance has two mainframes solely dedicated to Linux on z, running about 500 virtual Linux servers on them. They moved their Linux servers to the mainframe for the server consolidation benefits, but they kept their existing z/OS and Linux mainframes separate to "facilitate disaster recovery plan management," according to Gartner.
There can also be political reasons for keeping zLinux separate from z/OS mainframes, mainly due to what analyst Wayne Kernochan called the "graying of the old mainframe types." Kernochan, a senior research analyst at Aberdeen Group, said that those old mainframe types might be the z/OS experts. Once they retire, the business might migrate their z/OS applications to another platform, and so keeping them separate from zLinux apps might make sense.
"You might want to keep them separate, at least initially, to figure out how you want to deal with the decreased (z/OS) expertise," he said.
Technology benefits of combining z/OS and Linux
Even though IBM has touted a Linux-only mainframe, there are benefits to running both z/OS and Linux apps on the same box, said Kernochan.
First, the applications can work in concert with one another and communicate faster by being on the same hardware. For example, a Linux application might run a Web-facing program for a user that taps into a back-end database application running on z/OS. Housing both applications on the same hardware can speed communication and minimize security concerns.
"You're getting the benefit of using one system instead of two [or more]," Kernochan said. "You don't have to communicate between two systems or walk between them, so administrative costs can be lower."
But what about sharing resources on the same box? Is there concern that Linux apps could hog mainframe resources from z/OS, or vice versa? Not anymore.
"If you asked me that 10 years ago, I might give you a different answer," Kernochan said. "They've been redesigned. The architecture is now flexible enough to do that."
Kernochan elaborated, saying that 10 years ago there was still concern about running different operating systems on the same box. For instance, if the two virtual machines had to communicate, users worried it would increase the vulnerability of the system to outages. But since then, z/VM has matured such that virtual instances are treated as physical machines and resources aren't shared when they shouldn't be, he said.
Finally, Porell said there is a storage benefit to running z/OS and Linux on the same box. In particular, zLinux can take advantage of the hot-failover capabilities of z/OS, because z/OS can also manage the data of a Linux environment on the mainframe. "So it's instantly available should the system fail," he said.
The final benefit of mixing z/OS and Linux apps on the same box goes back to the reason why the two are usually kept separate: politics, Kernochan said.
"It exposes the two sides to each other," he said. "The more people that you have that are familiar with both, the happier you are. If they're on the same system, it's sort of natural."