Shared memory storage requirements

Shared memory is a type of virtual storage that allows multiple address spaces to easily address storage that is common to Db2 address spaces. This memory resides above the 2 GB bar. Db2 also uses 64-bit common storage, which is shared by the entire LPAR, for control structures that need to be accessed while running processes in and out of the system services address space (ssnmMSTR), database services address space (ssnmDBM1), and ssnmDIST address space.

The shared memory object is created at Db2 startup, and the following Db2 address spaces for the subsystem (ssnmDIST, ssnmDBM1, ssnmMSTR, and Utilities) are registered with z/OS® to be able to access the shared memory object.

To define the size of the shared memory and the 64-bit common area, use the HVSHARE and HVCOMMON parameters of the IEASYSxx member in the parmlib concatenation.

Restriction: Ensure that you have defined a high enough value for HVSHARE to satisfy all component requests for shared memory within the z/OS image. The default value is 510 TB. Db2 12 requires 1 TB of contiguous 64-bit shared private storage for each subsystem. This storage is virtual, and it is controlled by the z/OS HVSHARE parameter in IEASYSxx. This storage is not backed at allocation, only as it is used.

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IFCID 0225 has statistics that you can use to determine the amount of currently defined storage and how much of it is currently allocated.

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