9.2 Running Linux on the mainframe

On the mainframe, there are three options for running Linux:

• Natively on the mainframe hardware

• In a logical partition (LPAR)

• As a guest operating system under z/VM

Of these, the most interesting for the server consolidation case is the z/VM option.

Using LPARs, you can define up to 15 partitions (with the S/390 and zSeries machines), each of which hosts a separate server. If an LPAR contains CPUs that can run any operating system (so called general purpose engines), any mainframe operating system can run in that partition. The hardware resources of the mainframe (CPU, I/O, and memory) are divided according to the definition of each LPAR. zSeries and S/390 hardware have the computing power coupled with adequate memory and I/O to support hosting multiple operating-system images at one time.

Using z/VM, virtualization goes further than with LPAR to allow you dynamic definition of CPU, memory, and I/O channels as well as real sharing of memory, channels, and disks. Hundreds of images can run as guest operating systems under VM, as illustrated by consolidating entire server farms to Linux under z/VM by ISPCompany.

Software pricing for operating systems such as z/OS, z/VM, or VSE/ESA is generally based on the processing power of the machine. Adding CPUs normally increases this processing capability and, therefore, software prices. This was an early inhibitor to using Linux on the mainframe. IBM introduced Integrated Facility for Linux (IFL) to allow you to add processing power for Linux without increasing software pricing for traditional operating systems and other software. IFL features are managed by PR/SM as one or more logical partitions with dedicated CPUs. In IFL LPARs, you can have Linux images only or Linux and z/VM. Using the IFL, you can provide capacity dedicated to Linux workloads, as shown in Figure 9-1.

Figure 9-1. An IFL feature can be used to run Linux workloads