A computer operating system that allows legacy applications to be run automatically with quality of service (QoS) guarantees matching required QoS performance levels. In accordance with the invention, files have QoS requirement attributes that can be set-by users. Additionally, users may interpose a requirement broker between a given legacy application and the operating system. The requirement broker may be in the form of a modified version of a library that is dynamically linked with applications at load time. The requirement broker intercepts certain system calls and automatically requests from the system QoS guarantees in accordance with the QoS requirement attributes of the accessed files, whether local or remote.