Processor storage amount

If more than 4 terabytes (4 TB) is defined to a z/OS LPAR, all memory beyond 4 TB is part of the 2 GB large frame area. For information about the large frame area and the associated LFAREA parameter, see z/OS MVS Initialization and Tuning Reference. Storage reconfiguration is disabled for systems with more than 4 TB of processor storage defined.

Table 1 shows the maximum amount of processor storage that is supported by z/OS for each of the IBM servers.
Table 1. Maximum amount of processor storage supported by z/OS
IBM server Maximum amount of processor storage per IBM Z server Maximum amount of processor storage per z/OS LPAR
Start of changeIBM z16™ machine type 3931 (model A01)End of change Start of change40.0 TBEnd of change Start of change16 TB, for z/OS V2R5 and laterEnd of change
Start of changeIBM z16™ machine type 3932 (all models)End of change Start of change16.0 TBEnd of change Start of change16 TB for z/OS V2R5 and laterEnd of change
Start of changeIBM z15™ machine type 8561 (model T01)End of change Start of change40.0 TBEnd of change Start of change16 TB for z/OS V2R5 and laterEnd of change
Start of changeIBM z15™ machine type 8562 (model T02)End of change Start of change16.0 TBEnd of change Start of change16 TB for z/OS V2R5 and laterEnd of change
IBM z14™ (all models) 32.0 TB 16 TB for z/OS V2R5 and later
IBM z14 ZR1 8.0 TB 8 TB for z/OS V2R5 and later
For the storage amounts:
  • 1 TB equals 1,099,511,627,776 bytes.
  • 1 GB equals 1,073,741,824 bytes.

Minimum processor storage amount for IPLing a z/OS system

For z/OS V2R5 with the IBM z16™, IBM z15™, or IBM z14 server, a minimum of 8 GB of processor storage is required to IPL. When running as a z/VM guest or on an IBM z Personal Development Tool (zPDT), z/OS V2R5 requires a minimum of 2 GB of processor storage.

To assist your installation with upgrading from z/OSStart of changeV2R3End of change, IBM provides a health check that issues a warning for a system that is configured with less than 8 GB of processor storage.

If you attempt to IPL z/OS V2R5 on an IBM z16™, IBM z15™ or IBM z14 server (including the ZR1) with less than the minimum amount of processor storage, z/OS issues a warning WTOR message during IPL. Continuing with less than the minimum amount of processor storage might impact the availability of your system. If you attempt to IPL z/OS V2R5 on an earlier IBM server, no warning message is issued.

For earlier combinations of z/OS systems and IBM server models, smaller amounts of processor storage might be required for IPL. The minimum amount of processor storage that is required to IPL a z/OS system depends on various factors, including the following:
  • Number of devices in the I/O configuration
  • Number of address spaces to be created
  • Types of applications to be supported
  • Products and subsystems to be supported.
In internal tests that were conducted at IBM, it was found that 256 MB of processor storage was the minimum amount required to IPL a z/OS V2R2 system. The test systems were IPLed to the point of initializing the JES2 subsystem, and had I/O configurations that were defined with the following devices:
  • 51168 direct access storage devices (DASD)
  • 16 graphics devices
  • 5127 channel-to-channel (CTC) connections
  • 52 tape drives.

An actual production system is likely to require more than 256 MB of processor storage for normal operations.

For any release of z/OS, it is recommended that you monitor your processor storage usage, and determine whether to add more for better execution of workloads. The minimum stated here is not a likely environment for many installations, however, it has been observed that many systems perform adequately with 8 - 16 GB of processor storage, depending on the workload and system definition. A good indication that you have chosen an acceptable processor storage amount for your system is if no paging occurs, or if the amount of paging that occurs is within your service level objectives.