Scale and Concurrency of Massive File System Directories

File systems store data in files and organize these files in directories. Over decades, file systems have evolved to handle increasingly large files: they distribute files across a cluster of machines, they parallelize access to these files, they decouple data access from metadata access, and hence they provide scalable file access for high-performance applications. Sadly, most cluster-wide file systems lack any sophisticated support for large directories. In fact, most cluster file systems continue to use directories that were designed for humans, not for large-scale applications. The former use-case typically involves hundreds of files and infrequent concurrent mutations in each directory, while the latter use-case consists of tens of thousands of concurrent threads that simultaneously create large numbers of small files in a single directory at very high speeds. As a result, most cluster file systems exhibit very poor file create rate in a directory either due to limited scalability from using a single centralized directory server or due to reduced concurrency from using a system-wide synchronization mechanism. This dissertation proposes a directory architecture called GIGA+ that enables a directory in a cluster file system to store millions of files and sustain hundreds of thousands of concurrent file creations every second. GIGA+ makes two contributions: a concurrent