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Scalable, Global Namespaces with Programmable Storage


Global file system namespaces are difficult to scale because of the overheads of POSIX IO metadata management. The file system metadata IO created by today’s workloads subjects the underlying file system to small and frequent requests that have inherent locality. As a result, metadata IO scales differently than data IO. Prior work about scalable file system metadata IO addresses many facets of metadata management, including global semantics (e.g., strong consistency, durability) and hierarchical semantics (e.g., path traversal), but these techniques are integrated into ‘clean-slate’ file systems, which are hard to manage, and/or ‘dirty-slate’ file systems, which are challenging to understand and evolve.

The fundamental insight of this thesis is that the default policies of metadata management techniques in today’s file systems are causing scalability problems for specialized use cases. Our solution dynamically assigns customized solutions to various parts of the file system namespace, which facilitates domain-specific policies that shape metadata management techniques. To systematically explore this design space, we build a programmable file system with APIs that let developers of higher layers express their domain-specific knowledge in a storage-agnostic way. Policy engines embedded in the file system use this knowledge to guide internal mechanisms to make metadata management more scalable. Using these frameworks, we design scalable policies, inspired by the workload, for (1) subtree load balancing, (2) relaxing subtree consistency and durability semantics, and (3) subtree schemas and generators.

Each system is implemented on CephFS, providing state-of-the-art file system metadata management techniques to a leading open-source project. We have had numerous collaborators and co-authors from the CephFS team and hope to build a community around our programmable storage system.

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