SMC-D usage of memory buffers

For z/OS® Communications Server, a DMB is a contiguous, 1 MB block of pinned 64-bit private storage. A DMB is created on z/OS, into which the peer node can write, and a DMB is created on the peer node, into which z/OS can write. A DMB must be registered with the ISM interface so that the storage is available to the remote peer. Each DMB is associated with one and only one SMC-D link.

The SMC-D peers are not required to assign DMBEs of the same size for the client and the server of a specific TCP connection. The peers can use one or more DMBs for the TCP connections that are using the same SMC-D link.

When an SMC-D link is first established, z/OS Communications Server allocates a basic set of three DMBs for the link. The DMBE sizes for the DMBs are not defined initially, but instead are determined by the needs of the TCP connections that use the SMC-D link. z/OS Communications Server uses DMBE sizes of 64 KB, 128 KB, 256 KB, and greater than 256 KB. The appropriate size for the TCP connection is selected based on the receive buffer size of the local application. When the application does not explicitly set the buffer size by using SETSOCKOPT(), the default value is determined by the value of the TCPRCVBUFRSIZE parameter on the TCPCONFIG statement.

More DMBs are allocated as needed to accommodate more TCP connections or different receive buffer sizes.