How well the file system performs depends on how it is organized. Because a mountable file system must reside on a single DASD volume, several file systems on a volume or too much activity in a single file system can cause DASD I/O response time to be a bottleneck. In addition, some file system locking is done on a mountable file system basis. For these reasons, each user should normally have a unique mountable file system.

Another consideration is the placement of files in the file system hierarchy. Files deep in the hierarchy require several lookups each time they are opened.