Abstract: Non-volatile memories (NVMs) provide lower latency and higher bandwidth than block devices. Besides, NVMs are byte-addressable and provide persistence that can be used as memory-level storage devices (non-volatile main memory, NVMM). These features change storage hierarchy and allow CPU to access persistent data using load/store instructions. Thus, we can directly build a file system on NVMM. However, traditional file systems are designed based on slow block devices. They use a deep and complex software stack to optimize file system performance. This design results in software overhead being the dominant factor affecting NVMM file systems. Besides, scalability, crash consistency, data protection, and cross-media storage should be reconsidered in NVMM file systems. We survey existing work on optimizing NVMM file systems. First, we analyze the problems when directly using traditional file systems on NVMM, including heavy software overhead, limited scalability, inappropriate consistency guarantee techniques, etc. Second, we summarize the technique of 30 typical NVMM file systems and analyze their advantages and disadvantages. Finally, we provide a few suggestions for designing a high-performance NVMM file system based on real hardware Optane DC persistent memory module. Specifically, we suggest applying various techniques to reduce software overheads, improving the scalability of virtual file system (VFS), adopting highly-concurrent data structures (e.g., lock and index), using memory protection keys (MPK) for data protection, and carefully designing data placement/migration for cross-media file system.