Fair-ZNS: Enhancing Fairness in ZNS SSDs Through Self-Balancing I/O Scheduling

The NVMe zoned namespace (ZNS) is a new type of storage interface, which divides logical address space into fixed-size zones, and each zone strictly follows a sequential write constraint with a write pointer. Owing to the sequential write constraint of the ZNS, I/O requests would not be scheduled arbitrarily like the traditional solid state drives (SSDs) with a block interface. When multiple applications concurrently access one ZNS SSD hardware, the constraint deteriorates I/O blocking and causes huge unfairness. To resolve the problem, we propose a self-balance I/O scheduling dedicated for ZNS SSDs, called Fair-ZNS, to balance the slowdown among multiple applications and ensure fairness. Fair-ZNS identifies the unfair requests by the maximum slowdown value, and violently schedules these requests into the head of the queues overcoming the sequential write constraint. To eliminate the negative effect of the violent scheduling, Fair-ZNS deploys a self-balancing coordinator to fine-tune the order of the requests. Comprehensive evaluations show that Fair-ZNS alleviates I/O blocking and reduces average waiting time by
$8.3\times $
, increases fairness by
$2.3\times $
, and decreases the max slowdown by
$5.1\times $
averagely when compared to the current ZNS SSDs.