Western Digital has introduced the industry's first 20TB hard drives that integrates an iNAND UFS embedded flash drive (EFD) to improve performance, reliability, and capacity. The company's OptiNAND architecture does not use 3D NAND memory for caching, but to store various metadata to enhance key characteristics of hard drives.
Western Digital's 20TB HDDs with OptiNAND technology are based on nine 2.2TB ePMR (energy-assisted perpendicular magnetic recording technology) platters, triple-stage actuator technology for more precise positioning of read/write heads, an iNAND UFS drive of unknown capacity that uses 3D TLC NAND memory, and the company's custom system-on-chip (SoC) that controls the drive as well as communication between the HDD and the EFD.
Modern hard drives store gigabytes of metadata on spinning media since it cannot be cost-effectively stored in local DRAM and serial NAND. HDDs store repeatable runout (RRO) metadata (the share of the position error signal that is repeatable for every spindle revolution) as well as write operations metadata at the track level to account for increased adjacent track interference (ATI). With OptiNAND, RRO and write operations are stored on the iNAND drive, which frees up space on the rotating media, faster metadata availability, and reduces the number of read/write metadata-related operations, which further improve performance (e.g., random read/write performance). Additionally, the EFD stores write operations at the sector level, which optimizes storage requirements and reduce the number of ATI refreshes to increase performance.
As areal density of modern HDDs increases, so does the amount of metadata that needs to be stored on the drive. Also, things like ATI are affecting performance of ePMR-based HDDs stronger than before (something that can be solved with HAMR or MAMR magnetic recording technologies that are designed to greatly improve signal quality or TDMR read heads that can read data more reliably). Therefore, moving metadata from rotating media and placing it on a flash-based drive makes a lot of sense.
In addition, the iNAND EFD can be used to store over 100MB of write cache data in case of emergency power off (EPO) event, which improves reliability of an OptiNAND-enhanced HDD. Normally, drives from Western Digital only store about 2MB of write cache data to serial flash. Furthermore, with an iNAND EFD onboard and appropriate firmware optimizations, HDDs with OptiNAND can reduce their latency.
From a host perspective, Western Digital's OptiNAND architecture-based HDDs should work just like other drives without NAND flash. To that end, at least some customers of the company will be able to install the new drives into existing machines assuming that their 3.5-inch bays can handle slightly higher power consumption of iNAND-enhanced HDDs. Keeping in mind that Western Digital's exascale customers tend to qualify their drives before deploying, expect the drives to start shipping in high volume only several months (or even quarters) down the road.
Western Digital says that its OptiNAND technology will be used across multiple generations of its upcoming HDDs, including those based on ePMR and its successors.
"With our IP and world-class development teams in HDD and flash, we are able to continuously push the boundaries of innovation to improve our customers’ storage infrastructure," said Siva Sivaram, president of Global Technology and Strategy, Western Digital. "We have had an extraordinary journey of HDD innovation. We changed everything with HelioSeal in 2013; were first to ship energy-assisted HDDs in volume in 2019; and now we’re going to lead again with OptiNAND technology. This architecture will underpin our HDD technology roadmap for multiple generations as we expect that an ePMR HDD with OptiNAND will reach 50TB in the second half of the decade."
The manufacturer does not say how significantly the addition of an iNAND EFD affects costs of its HDDs, but it is obvious that their bill-of-materials increases with an additional component and a high-performance SoC controller. Keeping in mind that Western Digital's OptiNAND architecture has a number of advantages over traditional HDD architectures, it is likely that the producer will charge a premium for these drives.