AOS: Adaptive out-of-order scheduling for write-caused interference reduction in solid state disks

Abstract

The read/write performance asymmetry of Solid State Disks (SSDs) remains a critical concern for read performance. Under a concurrent workload with a mixture of read and write requests, preceding write requests preempt available flash memory resource so as to block read requests, which we call write-caused interference. Hence, the read performance, which is often more critical than the write performance, can be significantly degraded. Unfortunately, state-of-the-art schedulers either are inefficient in improving the read performance or suppress the write performance. In this paper, we propose a novel scheduler at device level, called AOS, to mitigate the write-caused interference and maximize the read performance without sacrificing the write performance. Specifically, AOS designs a conflict detection module, to efficiently identify access conflicts among requests. Then, AOS adaptively dispatches as many outstanding requests as possible to a re-ordering set based on the detected conflicts to reduce the write-caused interference and improve the flash-level parallelism (FLP). Finally, AOS carefully re-orders the dispatched requests to reduce channel-level access conflicts and improve the system-level parallelism (SLP). Extensive experimental results show that AOS reduces, an average of 51% read latency and 45% write latency, compared to FIFO.