Informix onunload/onload to take a snapshot and then restore

I have an Informix database. I want to take a snapshot, make some changes, then restore the database from that snapshot. (If Informix supported nested transactions, I would be using transactions.) The database is of moderate size (it can run on a consumer laptop) and there is no replication. It is critical that this backup/restore procedure is as fast as possible (I would like to do a backup-modify-restore cycle several times per second if possible). Only a small part of the database would be modified between the backup/restore points. Reading around a bit, it sounds like onupload/onload commands are the way to go. I can't quite figure out how to use them. Can you please show me the way to take a snapshot of a running database, and restore that database after some changes? (All on one machine.) Or if there is a better way to achieve this effect? (Perhaps there is a way to fork the process and all its files in a copy-on-write manner and have the child process serve a clone of the database on a different port?) If so please let me know.