We've got some performance problems with a baan/informix installation.
After informix restart, the application is quite fast for few days.
But after some days buffer wait (and io wait at os-level) starts to increase.

I found a health_check script (oninit.com).
It gave me warning:
Buffers allocated to index data is dangerously high [97.68%].

The script took that from onstat -P :

# onstat -
Informix Dynamic Server Version 7.31.FC6 -- On-Line -- Up 12 days 03:26:28 -- 4641784 Kbytes
#onstat -P | tail -4
Data 1.69
Btree 97.68
Other 0.63


Is this bad?
Sounds quite odd to me that indexes are using all the buffers.