IBM Z

A single-frame IBM z15 mainframe. Larger capacity models can have up to four total frames. This model has blue accents, as compared with the LinuxONE III model with orange highlights.

An IBM z14 mainframe. It is distinguished from the LinuxONE model by the blue accents on the doors.

IBM Z^[1] is a family name used by IBM for all of its z/Architecture mainframe computers from the z900 on. In July 2017,

Architecture

The zSeries, zEnterprise, System z and IBM Z families were named for their availability – z stands for zero downtime. The systems are built with spare components capable of hot failovers to ensure continuous operations.^[2]

The IBM Z family maintains full backward compatibility. In effect, current systems are the direct, lineal descendants of System/360, announced in 1964, and the System/370 from the 1970s. Many applications written for these systems can still run unmodified on the newest IBM Z system over five decades later.^[3]

Virtualization

Virtualization is required by default on IBM Z systems. First layer virtualization is provided by the Processor Resource and System Manager (PR/SM) to deploy one or more Logical Partitions (LPARs). Each LPAR supports a variety of operating systems. A hypervisor called z/VM can also be run as the second layer virtualization in LPARs to create as many virtual machines (VMs) as there are resources assigned to the LPARs to support them. The first layer of IBM Z virtualization (PR/SM) allows a z machine to run a limited number of LPARs (up to 80 on the IBM z13). These can be considered virtual "bare metal" servers because PR/SM allows CPUs to be dedicated to individual LPARs. z/VM LPARs allocated within PR/SM LPARs can run a very large number of virtual machines as long as there are adequate CPU, memory, and I/O resources configured with the system for the desired performance, capacity, and throughput.^{[citation needed]}

IBM Z's PR/SM and hardware attributes allow compute resources to be dynamically changed to meet workload demands. CPU and memory resources can be non-disruptively added to the system and dynamically assigned, recognized, and used by LPARs. I/O resources such as IP and SAN ports can also be added dynamically. They are virtualized and shared across all LPARs. The hardware component that provides this capability is called the Channel Subsystem. Each LPAR can be configured to either "see" or "not see" the virtualized I/O ports to establish desired "shareness" or isolation. This virtualization capability allows significant reduction in I/O resources because of its ability to share them and drive up utilization.^{[citation needed]}

PR/SM on IBM Z has earned Common Criteria Evaluation Assurance Level (EAL) 5+ security certification,^[4] and z/VM has earned Common Criteria EAL4+ certification.^[5]

The KVM hypervisor from Linux has also been ported.