Chapter 5. System z virtualization 27
Running Linux as a guest of z/VM is designed to provide the capability of running hundreds to
thousands of Linux images while benefiting from the reliability, availability, scalability, security
and serviceability characteristics of System z servers. At the same time, it allows customers
to exploit the exceptional capabilities of z/VM virtualization.
5.4.1 z/VM helps control costs with virtualization and server consolidation
One IBM System z server running z/VM Version 5 can do the job of many distributed servers
scattered across the enterprise by hosting a variety of platforms such as Linux on System z,
z/OS, z/OS.e, z/VM, z/VSE, and z/TPF. With Linux on System z, the combination of the z/VM
hypervisor and IBM mainframes can address infrastructure simplification issues faced by
many large enterprises while benefiting from greater availability, scalability, virtualization,
security, and reliability. System z environments with z/VM provide flexibility and management
characteristics that can make it possible for customers to satisfy the requirements of an On
Demand Business by deploying new Linux servers in minutes.
The complexity of maintaining large numbers of distributed servers can be relieved with a
single IBM mainframe and can help to reduce costs by requiring less floor space, less energy
consumption, and less person power. Simplification of the network by using HiperSockets can
provide savings and reduce cabling, hubs, switches, and routers, as well as help to reduce
maintenance effort. For many companies, critical enterprise data and applications are
entrusted to System z environments. Using Linux on System z can enhance the value of
customer applications and data by allowing Linux applications to communicate with other
System z applications and access critical data where it resides. This can help to improve
responsiveness and reduce unnecessary duplication of data.
Multiple Linux systems on System z servers can be easily created and managed with z/VM.
Linux server images can share physical resources as well as programs and data and internal
high-speed communications. z/VM V5 supports Integrated Facility for Linux (IFL) processors,
the attractively priced hardware feature for Linux workloads available for System z. Linux on
System z supports the IBM z/Architecture® (64-bit) on the System z platforms.
5.4.2 How z/VM virtualization works
The heart of z/VM is a multi-programming, multi-processing operating system kernel known
as the Control Program (CP).
1
CP is the component of z/VM that creates and dispatches
virtual machines on the real System z hardware. CP supports hundreds of commands,
including the configuration of the virtual machine, and it lets customers change virtual
machine configurations nearly at will.
System z virtualization depends on a hardware instruction called Start Interpretive Execution
(SIE, pronounced
sigh). Start Interpretive Execution is a special instruction that z/VM (and
PR/SM) use to run virtual servers in their very efficient native mode except when z/VM or
PR/SM needs to get control back for handling special situations, such as page faults, I/O
channel program translation, privileged instructions, and so on. The result is that for most of
the time these logical/virtual servers run without any emulation overhead, making z/VM and
PR/SM highly efficient virtualization machines.
Virtualization of processors
Because z/Architecture defines that a System z data processing system can house one to 64
processors, each virtual machine z/VM creates can have one to 64 virtual processors. z/VM
provides control over processor resources by letting a system administrator assign a share
1
z/VM virtualization details provided from “Basics of z/VM Virtualization” by Bill Bitner and Brian Wade in the
z/Journal February/March 2008 edition.