A workload-aware I/O scheduler in a Software-Defined Hybrid Storage (SDHS) system is disclosed. The scheduler includes a queue managing module, a workload property database, a traffic monitoring module, and a traffic modeling module. The queue managing module further has a request receiving sub-module, a request controlling sub-module, and a request dispatching sub-module. With the operations of the queue managing module, requests for accesses of HDDs and SSDs can be separately queued. Thus, performance of the SDHS can be improved. Meanwhile, the scheduler can be used for requests from a number of workloads at the same time while meet performance requirement of each workload.