FusionRAID: Achieving Consistent Low Latency for Commodity SSD Arrays

The use of all-flash arrays has been increasing. Compared to their hard-disk counterparts, each drive offers higher performance but also undergoes more severe periodic performance degradation (due to internal operations such as garbage collection). With a detailed study of widely-used applications/traces and 6 SSD models, we confirm that individual SSD's performance jitters are further magnified in RAID arrays. Our results also reveal that with SSD latency low and decreasing, the software overhead of RAID write creates long, complex write paths involving more drives, raising both average-case latency and risk of exposing worst-case performance.

Based on these findings, we propose FusionRAID, a new RAID architecture that achieves consistent, low latency on commodity SSD arrays. By spreading requests to all SSDs in a shared, large storage pool, bursty application workloads can be served by plenty of ''normal-behaving'' drives. By performing temporary, replicated writes, it retains RAID fault-tolerance yet greatly accelerates small, random writes. Blocks of such transient data replicas are created in stripe-ready locations based on RAID declustering, enabling effortless conversion to long-term RAID storage. Finally, using lightweight SSD latency spike detection and request redirection, FusionRAID avoids drives under transient but severe performance degradation. Our evaluation with traces and applications shows that FusionRAID brings a 22%–98% reduction in median latency, and a 2.7x–62x reduction in tail latency, with a moderate and temporary space overhead.