INFORMIX-OnLine is the follow-up and enhancement to the Standard Engine product.
OnLine is a continuously running program on a main computer; this program is known as a database server or an engine. This engine controls how the data is stored, retrieved, physically maintained on the computer, and presented logically to users—all done within the rules of the relational database model. Therefore, Informix's database server engine is actually a relational database management system (RDBMS).
INFORMIX-OnLine achieves better performance by building its own I/O and multiprocessor routines (DMA).
Lack of a built-in wait queue in OnLine.