Mainframe resource Sharing

Almost all sites with an IBM Z mainframe will run multiple operating systems .
Those might be separate dev,test and prod images, or different systems for different business units. Or just multiple systems for resilience.

One mainframe CPC will run a few operating systems
Those can be

Each of those will need resources such as CPU memory and I/O and share it within the CPC

The PR/SM system is used to share the resources in this manner. it comes pre-installed on IBM mainframes.

PR/SM is the name of the system that the Mainframe CPCs use to share resources between each other.
You can think of it as kind of a hypervisor like VMWare or Hyper-V

it divides the CPC into logical partitions LPARs, giving one per operating system.

The CPU assignment can also be physical, rather than logical, although if an LPAR does not need a CPU that was physically assigned to it, other LPARS won't be able to make use of it.

If shared logically, each LPAR will believe that it has exclusive access to the CPU, although it is shared.

Within this system you can assign different weights to different LPARS, dictating which ones will have to wait less than the "lighter" ones for the shared CPUs

Different types of processors can be shared and managed with the use of this system

System Recovery Boost

Since the release of z15, the System Recovery Boost function was introduced to allow temporary zIIP capacity to be utilised during a shutdown or IPL period to speed up the recovery times and minimize downtimes.

The extra zIIP capacity is then shared across LPARs according to how this system was configured in the management controls.
An LPAr needs to be configured with predefined reserved logical zIIP capacity before they can be used during this boost period

The CSS (Channel SubSystem) channels can also be shared using the Multiple Image Facility

OSA (Open Systems Adapter)s can also be shared with the use of this system, again with each system thinking they have exclusive access.

Logical PARtiton is a "slice" of a mainframe system created by the PRSM system, each instance of an operating system will receive one LPAR. A z16 mainframe can have up to 85 LPARs

Each one will be assigned a smaller subset of the systems resources, for example if you have 15 CPUs and 600 GB of ram you can assign 5 CPUs and 200GB of ram to each of the 3 LPARS (though you can divide them how you want).

If the CPU wanted by another LPAR is busy, the LPAR will have to wait until the CPU frees up, or the PRSM decides it's CPU time is up.

This operating system can be used as a hypervisor.

It operates within one LPAR itself, and it can host guests each of them having and running their own operating system,

Although those guest systems won't have their own LPAR z/VM can provide the same features as PRSM, allowing the guests to share CPU, memory, CSS (Channel SubSystem)s and OSA (Open Systems Adapter) adapters.

it is common that z/VM will be used to host Linux on IBM Z partitions.

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