The onstat -g iob option prints a summary of big buffer use.
IBM Informix Dynamic Server Version 10.00.UC1 -- On-Line -- Up 01:55:13 -- 101376 Kbytes
AIO big buffer usage summary:
class reads writes
pages ops pgs/op holes hl-ops hls/op pages ops pgs/op
fifo 0 0 0.00 0 0 0.00 0 0 0.00
kio 0 0 0.00 0 0 0.00 0 0 0.00
adt 0 0 0.00 0 0 0.00 0 0 0.00
msc 0 0 0.00 0 0 0.00 0 0 0.00
aio 0 0 0.00 0 0 0.00 607 607 1.00
pio 0 0 0.00 0 0 0.00 0 0 0.00
lio 0 0 0.00 0 0 0.00 0 0 0.00