Are there any Informix-specific guides for optimizing any operating system where an ifx engine is running? For example, in Linux, strip-down to a bare minimum all unecessary binaries, daemons, utilities, tune kernel parameters, optimize raw and cooked devices (hdparm). Someday, maybe, informix can create its own proprietary PICK-like O/S. The general idea is for the OS where ifx sits on have the smallest footprint, lowest overhead impact on ifx and provide optimized ifx performance.
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Give IDS more disk spindles, more main memory, more CPUs, less contention from other software, and it will generally work better. (Funny that - sounds like a recipe for good performance for any software that can actually exploit multiple CPUs.)
One of the trickier issues is weighing kernel buffer pool against Informix's preference for direct I/O. Generally speaking, a DBMS will work best when the O/S is not doing much buffering for it; the DBMS is already handling buffering in shared memory and wants its writes to go direct to disk. So you want to tune the kernel buffer pool downwards, and let IDS use the space saved.
Minimizing the run-time footprint of the o/s support services is a good idea; removing superfluous services increases security and reduces workload on the machine, leaving more for IDS. You have to decide what services you do need. If all the clients are connecting from off the box, you probably need SSH for the admins to connect to/with, FTP, SNTP, maybe SNMP (but probably not), the network services (DNS, any network authentication - YP, NIS+ or LDAP or what have you), and not a lot else.