Setting the size of the receive-any input areas
The system initialization parameter, RAMAX, specifies the size in bytes of the I/O area that is to be allocated for each SNA receive-any operation. You can use the RAMAX system initialization parameter in any networks that use the z/OS® Communications Server SNA access method for LUs.
These storage areas are called receive-any input areas (RAIAs) and are used to receive the first terminal input for a transaction from the SNA. All input from SNA comes in request/response units (RUs).
Storage for the RAIAs, which is above the 16 MB line, is allocated by the CICS® terminal control program during CICS initialization. This storage remains allocated for the entire execution of the CICS job step. The size of this storage is the product of the RAPOOL and RAMAX system initialization parameters.
Effects
SNA attempts to put any incoming RU into the initial receive-any input area, which has the size of RAMAX. If this area is not large enough, SNA creates a message indicating the problem and stating how many extra bytes are waiting that cannot be accommodated.
RAMAX is the largest size of any RU that CICS can take directly in the receive-any command. It is a limit against which CICS compares the indication from SNA of the overall size of the RU. If there is more, it is saved by SNA, and CICS gets the rest in a second request.
With a small RAMAX, you reduce the virtual storage taken up in RAIAs. However, you risk more processor usage in SNA tries again to get any data that could not fit into the RAIA.
For many purposes, the default RAMAX value of 256 bytes is adequate. If you know that many incoming RUs are larger than this value, you can always increase RAMAX to suit your system.
For individual terminals, there are separate parameters that determine how large an RU is going to be from these devices. It makes sense for RAMAX to be at least as large as the largest SENDSIZE attribute for frequently used terminals.
Limitations
Real storage can be wasted with a high RAMAX value. If the RAMAX value is set too low, extra processor time is needed to acquire additional buffers to receive the remaining data.
Suggestions
Set RAMAX with the size in bytes of the I/O area allocated for each receive-any request issued by CICS. The maximum value is 32767. Because most inputs are 256 bytes, this size is the default value specified.
Set RAMAX to be slightly larger than your CICS system input messages. If you know the message length distribution for your system, set the value to accommodate most of your input messages.
In any case, the size required for RAMAX need only take into account the first (or only) RU of a message. Thus, messages sent using SNA chaining do not require RAMAX to be set based on their overall chain length, but only on the size of the constituent RUs.
Do not specify a RAMAX value that is less than the RUSIZE (from the CINIT) for a pipeline terminal because pipelines cannot handle over-length data.
Receive-any input areas are taken from a fixed-length subpool of storage. A size of 2048 might appear to be adequate for two such areas to fit on one 4 KB page, but only 4048 bytes are available in each page, so only one area fits on one page. Defining a size of 2024 ensures that two areas, including page headers, fit on one page.
Monitoring
The size of RUs or chains in a network can be identified with an SNA line or buffer trace.