Abstract: NAND flash is widely utilized in mobile devices due to its excellent characteristics, including large capacity, light weight, and shock resistance. The flash friendly file system (F2FS), designed for flash features, is a typical log-structured file system (LFS). It employs a log-structured write mechanism to enhance random write performance, utilizes roll-forward recovery technology for fast consistency protection, and is commonly used as a file system for mobile devices. However, the performance of file system is impacted by fragmentation and segment cleaning. The out-of-place update mechanism of LFS and the small write mode of high-concurrency and random synchronization of mobile applications exacerbate fragmentation, leading to sluggish I/O request responses and device operation freezes. We initially introduce the relevant concepts and content of log-structured file systems in mobile devices. We then primarily outline the research status of fragmentation and segment cleaning of LFS. Firstly, we analyze the generation and impact of fragmentation, summarize the research work on reducing fragments from the perspectives of preventing fragments and reorganizing fragments. Secondly, we examine the impact of the mixed storage of hot and cold data on segment cleaning. Additionally, we summarize the research status of distinguishing hot and cold data from static and dynamic classification, and segment cleaning from the perspectives of managing data distribution and adjusting the timing, frequency, and objects of segment cleaning. Finally, we outline the main challenges and future research prospects of log-structured file systems in mobile devices.