AIS: An Active Idleness I/O Scheduler to Reduce Buffer-Exhausted Degradation of Solid-State Drives

Modern solid-state drives (SSDs) continue to boost storage density and I/O bandwidth at the cost of flash-access I/O latency, especially for write, hence prevalently deploy a build-in buffer to absorb incoming writes. However, when the buffer is used up, the applications suffer from a sudden and long performance declines, i.e., buffer-exhausted degradation (BED). To holistically understand BED and recovery, we design an automated testing toolset (SSDTest) to measure six commodity NVMe SSDs and find: 1) the occurrence of the BED strictly relies on the written-data amount, 2) BED dramatically increases I/O latency of SSDs, especially write and read-after-write, 3) BED can be conditionally reduced and recovered only after a period of idle time, and 4) a read without preceding writes is largely immune to BED, but prolongs the required idle time to recover the available buffer. Furthermore, we build a black-box SSD buffer-recovery model to quantitatively characterize the idleness-recovery behaviors and design an SSD BED predictor to make BED occurrence and buffer recovery predictable. Leveraging this model, we further design an Active Idleness I/O Scheduler (AIS) with small-sized auxiliary storage to actively regulate the I/O idle-intervals to maximize the internal buffer recovery of SSD. AIS adaptively steers incoming data to the auxiliary storage to 1) strategically keep SSD idle to reduce the occurrence of BED and 2) mitigate the tail latency of SSDs caused by read-after-writes during BED. We perform extensive evaluations under a variety of workloads. The results show that AIS improves average, 99th, 99.9th and 99.99th-percentile latencies of SSDs by up to 29.3%, 37.3%, 78.7% and 67.2% respectively, with up to 512MB auxiliary storage.