Metadata of file systems is the data in charge of maintaining namespace, permission semantics and location of file data blocks. Metadata operations can account for up to 80% of total file system operations. As such, the performance of metadata services substantially impacts the overall performance of distributed file systems, especially with the advent of big data era, posing great pressure on the underlying storage systems. This paper reports the state-of-the-art research on the metadata services in large-scale distributed file systems. The study was conducted from three perspectives that are always used to characterize the file systems: high scalability, high-performance and high-availability, with focus squarely on their respective major challenges as well as their developed mainstream technologies. Additionally, some existing problems in this research were also identified and analyzed, which could be used as a reference for related studies.