Overcoming a Zone Reclaiming Overhead with Partial-Zone Reclaiming

Inho Song, Wonjin Lee, Jaedong Lee, Seehwan Yoo, Jongmoo Choi

http://doi.org/10.5626/JOK.2024.51.2.115

Solid State Drive (SSD) suffers unpredictable IO latency and space amplification due to the traditional block interface. Zoned Namespace, which is a more flash friendly interface, replaced the block interface bringing reliable IO latency and increasing both the capacity and lifespan of SSDs. The benefit of the zone interface is not free. A Zoned Namespace (ZNS) SSD delegates the garbage collection and data placement responsibility to the host, which requires host-level garbage collection called "zone reclaiming". At the same time, ZNS SSD exposes a larger zone to the host to exploit the device parallelism. The increased number of blocks to a zone gives high parallelism; however, the overhead of the zone reclaiming process becomes high with the increased size of the zone. Eventually, the host neither expects predictable latency nor optimal performance due to the background process. This paper tackles the overhead of the zone reclaiming process by introducing "Partial Zone Reclaiming" method. Partial zone reclaiming delays the ongoing reclaiming process and handles the host request that is on the fly. In our experiment, partial zone reclaiming not only improved the host request latency by up to 8% on average, but also reduced zone reclaiming time by up to 41%.