Buffer sizes for physical links

Use at least the size of the largest message segment that is to be transmitted across this physical link. The buffer sizes at each end of a physical link must be equal.

Specify a buffer size with the BUFSIZE keyword on either the CREATE MSPLINK command or the MSPLINK macro.

You can specify different buffer sizes for individual logical links by using either the IMS type-2 command UPDATE MSLINK NAME(linkname) SET(BUFSIZE(new_bufsize)) or the IMS type-1 command /UPDATE MSLINK NAME(linkname) SET(BUFSIZE(new_bufsize)).

To update the buffer sizes for all logical links that are assigned to a physical link, use the command UPDATE MSPLINK NAME(linkname) SET(BUFSIZE(new_bufsize)).

See the description of the BUFSIZE keyword for the minimum and maximum sizes that you can specify for link buffers.

You can display MSC statistics to help determine the optimum buffer size. MSC statistics are displayed by the IMS type-2 command QUERY MSLINK NAME(linkname) SHOW(STATISTICS). Use the statistics Hi_Msg_Send_SZ and Hi_Msg_Rec_SZ to see the maximum message sizes being sent and received. You can also compare the number of send and receive I/O requests to the number of messages that are sent and received. If IMS requires multiple send and receive I/O requests to send a single message, the buffers are too small. The send and receive I/O requests are captured by the statistics Tot_Send_CT and Tot_Rec_CT. The number of messages sent and received are captured by the statistics Tot_Msg_Send_CT and Tot_Msg_Rec_CT.

Depending on the physical link type, storage for the link buffer sizes are allocated from different pools. TCP/IP and VTAM® link buffers are allocated from the high input/output pool (HIOP). CTC link buffers are allocated from the communications input/output pool (CIOP). MTM link buffers are allocated from subpool 231 in the common storage area (CSA).