Systems, methods, apparatuses, and software for data storage systems are provided herein. In one example, a data storage apparatus includes a processor configured to assign service levels in a queue for handling storage operations directed to one or more data storage drives. The processor can pre-allocate resources in the queue for selected ones of the service levels before ones of the storage operations associated with the selected ones of the service levels are received by the processor. The processor receives the storage operations, and based at least on the service levels, services the storage operations from the queue with the one or more data storage drives.