Land of Oz: Resolving Orderless Writes in Zoned Namespace SSDs

Abstract

Zoned Namespace (ZNS) SSDs present a new class of storage devices that promise low cost, stable performance, and software-defined capability. ZNS abstracts the SSD into an array of zones that can only be written sequentially. Although ZNS-compatible filesystems (e.g., F2FS) launch sequential writes, the zone write constraint may be violated due to orderless Linux I/O stack. Hence, the write queue depth of each zone must be limited to one, which severely degrades the performance of small writes.

In this paper, we propose oZNS SSD (o: orderless), which allows multiple writes to be submitted to a zone and processed out-of-order in the SSD. To bridge the gap between ZNS sequential write contract and orderless writes on flash, a lightweight indirection layer is introduced and carefully designed by exploiting the characteristics of out-of-order writes. Specifically, memory-efficient metadata structures are devised to record the write deviations of data pages, and a two-tier buffering mechanism is employed to reduce metadata and accelerate metadata access. Moreover, a read-try policy removes metadata access from read critical path and thus improves the data read efficiency. Experimental results show that oZNS can achieve up to 8.6$\times$× higher write performance than traditional ZNS while providing comparable read performance.