What We’ve Learned after 6 Years of IO Scheduling

The blog post by Pavel "Xemul" Emelyanov discusses the intricacies of IO scheduling in ScyllaDB, emphasizing the importance of prioritizing IO requests to ensure timely processing, especially in latency-sensitive OLTP workloads. ScyllaDB utilizes the Seastar framework's IO scheduler, which manages request priorities and dispatches tasks into multiple queues, ultimately optimizing them for processing by the Linux kernel and disk. The scheduler's critical parameter, the "latency goal," guides the system in managing concurrency levels to ensure requests are completed within a specified time frame, despite challenges posed by varying disk performance characteristics. Tools like iotune are employed to measure disk performance parameters in advance, aiding in accurate scheduler configuration. The post also delves into handling "pure," "mixed," and "unstable" workloads, highlighting how request size and type affect disk throughput and detailing techniques for maintaining performance under different conditions. The article underscores the complexity of modeling disk behavior due to its unpredictable nature and emphasizes the need for a nuanced approach to configuring IO schedulers for optimal performance.