Abstract: Programmable network devices, represented by programmable switches and SmartNICs, are increasingly used in modern data centers to support the execution of customized data processing logic on network data transmission paths, which brings new opportunities for building high-performance in-network storage systems. However, programmable network devices have hardware resource limitations (e.g., limited expressive powers and small memory space), and there are still many challenges to fully utilize their advantages and maximize the acceleration of storage systems. We systematically review the recent research progress of in-network storage systems. First, we describe the hardware architecture and performance characteristics of programmable network devices, and based on this, we summarize two major challenges in building high-performance in-network storage systems: 1) division of labor between hardware and software, 2) fault tolerance of the storage systems. Then, according to the tasks performed by programmable network devices (data caching, distributed coordination, request scheduling, data aggregation), we classify and describe existing in-network storage systems. Moreover, using several examples of in-network storage systems, we analyze corresponding design difficulties and software technologies. Finally, we indicate open problems that need to be explored in further research on in-network storage systems, including switch-NIC collaboration, data security, multi-tenancy, and automatic function offloading.