QDLS is a relic from the past. It was IBM’s first attempt to encorporate a PC- or Unix-like directory system in OS/400. These days OS/400 has the IFS (Integrated File System), which does it the other way round: it encorporates the original library system in a Unix-like directory system.
QDLS has several disadvantages (compared to a directory in the root file system):
- To use it a user has to be enrolled in the system directory
- File names are limited to 8.3 (FILENAME.EXT)
- It has more overhead and thus is slower