There is a critical problem of metadata performance scalability in high-performance computing when accessing many small files from millions of clients. This paper proposes a design of PPMDS, a scalable distributed metadata server, to improve metadata performance and scalability. To efficiently manage hierarchical namespace of file system across multiple servers, it manages inode entries using a parent inode number and an entry name as a key. To update multiple inode entries atomically, it uses a nonblocking distributed transaction based on an algorithm of dynamic software transactional memory. It improves the metadata performance by introducing server-side transaction processing, multi-reader, and a shared lock mode to reduce the number of remote procedure calls and to prevent unnecessary blocking. A prototype implementation achieves 62,000 operations per second when creating files in a single directory using 8 metadata servers, which is 2.58x performance improvement compared to a single metadata performance.