If an operating system supports forced residency of the Informix resident portion of the shared memory, you may opt to keep this chunk of memory always resident in operating system's physical memory, by specifying RESIDENT 1, this signifies that it will not be swapped to disk. (default is 0=no)
Before setting this to 1, you must see what is the current resident size of your instance(s) by:
onstat -g seg
look at the class column and look for class type "R". You must make sure that you have left with sufficient memory after deducting the resident portion of the memory, for operating system need and other processes.