Most of the system memory that z/OS® Communications Server uses in support of SMC-R is allocated from 64-bit virtual and real memory, and is page-fixed. The amount of fixed memory that Communications Server requires for SMC-R communications is a combination of storage that is required to operate the IBM® 10GbE RoCE Express® interfaces and storage that is required to handle SMC-R workloads.
This storage is primarily for staging buffers and is obtained when the first 10GbE RoCE Express interface is started. Additional staging buffers are obtained depending on SMC-R usage by this host.
This storage primarily represents the RMB storage that is allocated for the SMC-R link groups that are created. The number of SMC-R link groups that are created is influenced by the number of physical LANs and virtual LANs (VLANs) that are supported, and by the number of remote SMC-R capable peers.
More storage is required depending on the number and characteristics of TCP connections that are using SMC-R communications. For example, the TCP connection socket send and receive characteristics and the socket send and receive activity influence the amount of storage that the stack requires for SMC-R.
You can limit the amount of storage that a TCP/IP stack uses for SMC-R by specifying the FIXEDMEMORY subparameter of the SMCR parameter on the GLOBALCONFIG profile statement. The default value for the FIXEDMEMORY subparameter is 256 MB. If the TCP/IP stack reaches the FIXEDMEMORY limit, new TCP connections cannot use SMC-R communications and instead use traditional TCP/IP communications.
The FIXEDMEMORY subparameter does not control the amount of VTAM storage that is allocated to operate the 10GbE RoCE Express interfaces. You must still consider the VTAM storage requirements, however, when you are determining how much fixed storage is required for SMC-R.