Zoned storage devices are a paradigm shift in the way we view storage
today for both hard disk and solid state drives. A zoned storage device divides the LBA space into zones (i.e., large and contiguous address
ranges) that can only be written sequentially. This breaks long-standing assumptions associated with the traditional block interface that has been the undisputed storage interface since the early days of UNIX. Zoned storage devices are governed by a “zone interface”---Zoned Namespaces (ZNS) in NVMe SSDs and ZBC/ZAC in Shingled Magnetic Recording HDDs---a new interface defining a novel division of responsibilities between storage software and device firmware. We are exploring methods for adapting and redesigning storage software, such as key-value stores, file systems, and distributed storage systems, to make them compatible with zoned devices with the least amount of effort.
Despite significant technological differences, emerging data center solid state drives (SSDs) and hard disk drives (HDDs) both fit the zone interface. Through ZNS, SSDs can significantly narrow the scope of the Flash Translation Layer (FTL), which has been the bane of SSDs’ existence by introducing performance unpredictability in the form of long tail latencies and significant write amplification. ZNS helps eliminate page-based remapping and garbage collection from the FTL by matching the aforementioned zones to groups of underlying NAND erase blocks. By virtually eliminating garbage collection, this greatly reduces write amplification and the need for capacity overprovisioning, which in turn reduces device cost. On the other hand, HDDs now boost their capacity using Shingled Magnetic Recording (SMR), which divides the drive surface into groups of tracks that must be written sequentially. ZBC/ZAC exposes these groups of tracks to the host as individual zones, avoiding the need for a translation layer similar to SSDs’ FTL. Because of the general applicability of the zone interface, we believe that many fundamental aspects of software for zoned storage will work across both types of devices.
We thank the members and companies of the PDL Consortium: Alibaba Group, Amazon, Datrium, Facebook, Google, Hewlett Packard Enterprise, Hitachi Ltd., Intel Corporation, IBM, Microsoft Research, NetApp, Inc., Oracle Corporation, Pure Storage, Salesforce, Samsung Semiconductor Inc., Seagate Technology, Two Sigma, and Western Digital for their interest, insights, feedback, and support.