Communications storage manager (CSM) is a component of VTAM® that allows authorized host applications to share data with VTAM and other CSM users without having to physically pass the data. A CSM user can be any system-authorized application program or product.

CSM is provided as part of the high-performance data transfer (HPDT) family of services. HPDT optimizes system performance for the transfer of bulk data. By providing a means for authorized applications to share buffers, CSM improves system performance during the transfer of bulk data by reducing the processing required for data movement. As a result, CPU resources (CPU cycles, memory bus, and cache) are conserved.

VTAM uses CSM to perform channel I/O over an HPDT MPC connection and to provide the HPDT services for host LU 6.2 applications. (HPDT MPC connections are explained in Multipath channel connections.) Host LU 6.2 applications use the IVTCSM and APPCCMD macroinstructions to reduce the use of system resources for large data transfers. For more information about HPDT services, see High-performance data transfer (HPDT).

CSM includes an application programming interface (IVTCSM macroinstruction) that allows users to obtain and return storage in the form of CSM buffer pools by using the IVTCSM macroinstruction. Applications must be authorized to use the IVTCSM macroinstruction.

CSM buffer pools are identified by the fifteen distinct combinations of storage type and buffer size as described in Table 1 and specified on the IVTCSM REQUEST=CREATE_POOL macroinstruction.

Data space storage is a common area data space and is associated with the master scheduler address space. This association results in a data space that persists for the life of the system.

Data space storage is either 31-bit backed or 64-bit backed. 31-bit backed data space, when fixed, resides below the 2-gigabyte real storage bar. 64-bit backed data space, when fixed, can reside below or above the 2-gigabyte bar. Where the storage is backed is a concern for only those products performing I/O into or out of the storage.

The application programming interface for CSM is described in z/OS Communications Server: CSM Guide.

The rest of this appendix summarizes the interfaces that you can use to install, configure, monitor, and diagnose CSM storage.