Zoned Namespace is a Double-edged Sword: Rethinking the Impact of the ZNS Interface on File System Scalability

Abstract

Emerging Zoned Namespace (ZNS) SSDs transfer garbage collection (GC) to the host, thereby providing more stable performance and extending device life. Although the ZNS SSD has these hardware advantages, each zone in the ZNS SSD must be written sequentially. File systems always need to implement additional mechanisms (IO Scheduling or modifications to the driver/interface) to meet the sequential write constraint of the ZNS SSD, while fully utilizing the high bandwidth of the ZNS SSD under the constraint. These additional mechanisms usually bring extra overhead or limitations and can also cause the scalability problem for the file system. Therefore, in the past, it was naturally assumed that the sequential write constraint of the ZNS SSD would complicate the design of the file system and have a negative impact on file system scalability. However, in this paper, we argue that the previous insight on the impact of the ZNS SSD on file systems is not comprehensive enough. We regard the ZNS interface as a double-edged sword for file system scalability and found that the sequential write constraint of the ZNS device also has a positive impact on file system scalability. Specifically, order-preserving write is essential for crash consistency in file systems but is the scalability bottleneck of file systems that are compatible with SSDs that support Zoned Namespaces. The analysis in this paper indicates that the sequential write constraint of the ZNS device provides an excellent opportunity to simplify and optimize the order-preserving write mechanism in file systems. Based on the new insight, we propose a scalable ZNS-compatible file system called ScaleZNFS. Contrary to previous work that only focuses on mitigating the limitations imposed by the ZNS interface on the file system, ScaleZNFS not only mitigates the negative impact of the ZNS interface on file system scalability through its multi-head logging design and the per-log IO scheduler, but also employs the ZNS-driven order-preserving write mechanism, which can leverage the sequential write constraint of the ZNS interface, to eliminate the scalability bottleneck associated with the order-preserving write. ScaleZNFS outperforms F2FS and XFS by up to 5.05 × and 2.94 ×, respectively, and it can maintain stable scalability with increasing threads.