As buffer pools expand, expansion uses virtual storage that can potentially impact an operating system that has virtual storage constraints. VTAM® use of virtual storage can be limited by the CSA start options and the virtual storage allocated to VTAM address space.

All buffer pools are defined in virtual storage above the 16 MB address line in extended virtual storage. Therefore, the size of the buffer pools does not usually impact virtual storage constraints.

You can use the SONLIM start option to limit the amount of fixed I/O buffer storage available for session outage notification messages. This controls VTAM use of fixed (real) storage. The default (60 percent) should be sufficient for most environments. This percentage is calculated from the base number of buffers specified when VTAM is started but does not include buffers from dynamic buffering in its calculations. Therefore, the default value of 60 percent is normally sufficient if you do not preallocate most of your I/O buffers at start time and if you use dynamic buffering. If you preallocate most of your I/O buffers at start time, you should consider overriding the VTAM default with a smaller percentage.