Nowadays more and more applications require file systems to efficiently maintain million or more files. How to provide high access performance with such a huge number of files and such large directories is a big challenge for cluster file systems. Limited by static directory structures, existing file systems will be prohibitively inefficient for this use. To address this problem, we present a scalable and adaptive metadata management system which aims to maintain a trillion files efficiently. Firstly, our system exploits an adaptive two-level directory partitioning based on extendible hashing to manage very large directories. Secondly, our system utilizes fine-grained parallel processing within a directory and greatly improves performance of file creation or deletion. Thirdly, our system uses multiple-layered metadata cache management which improves memory utilization on the servers. And finally, our system uses a dynamic loadbalance mechanism based on consistent hashing which enables our system to scale up and down easily. Our performance results on 32 metadata servers show that our user-level prototype implementation can create more than 74 thousand files per second and can get more than 270 thousand files' attributes per second in a single directory with 100 million files. Moreover, it delivers a peak throughput of more than 60 thousand file creates/second in a single directory with 1 billion files.