from onstat -g ath output
grep for sm_read
informix@xxxxxxx: onstat -g ath | grep sm_read
23519 5d7ce028 5aa622d0 1 cond wait sm_read 8cpu sqlexec
To trace back the session
grep the 3rd column with onstat -u
informix@devlx005: onstat -u | grep 5aa622d0
5aa622d0 Y–P— 21045 informix 6 5bc6bd48 0 1 0 0
Session ID = 21045