Scalable Write Allocation in the WAFL File System

Enterprise storage systems must scale to increasing core counts to meet stringent performance requirements. Both the NetApp
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Data ONTAP
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storage operating system and its WAFL
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file system have been incrementally parallelized over the years, but some components remain single-threaded. The WAFL write allocator, which is responsible for assigning blocks on persistent storage to dirty data in a way that maximizes write throughput to the storage media, is single-threaded and has become a major scalability bottleneck. This paper presents a new write allocation architecture, White Alligator, for the WAFL file system that scales performance on many cores. We also place the new architecture in the context of the historical parallelization of WAFL and discuss the architectural decisions that have facilitated this parallelism. The resulting system demonstrates increased scalability that results in throughput gains of up to 274% on a many-core storage system.