Doubts with ISM and ON-Bar

Dear Friends: I am evaluating backing up my database with Informix Storage Manager and ON-Bar in disk files, and there are some things we would like to consult them, such as: 1 - I see the ON-Bar generates a maximum of 2GB files, is this normal, or I have to change some parameter? 2 - If I have the LTAPEDEV variable set to NUL in the ONCONFIG, will I have problems to backup ?, what if I do not do backup of the logs might have difficulties? 3 - If I had to change the above parameter, what information would ?. My engine is an IDS 7.31 TD6, running on a Windows 2000 backup to disk I do because I do not get the tape recorder (Ultrium LTO II). Of course, I appreciate your kind attention.