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1. Asymmetric RAID: Rethinking RAID for SSD Heterogeneity

   https://doi.org/10.1145/3655038.3665952  

   Jiao, Ziyang; Kim, Bryan S (July 2024, ACM)

   Traditional RAID solutions (e.g., Linux MD) balance writes evenly across the array for high I/O parallelism and data reliability. This is built around the assumption that the underlying storage components are homogeneous, both in performance and capacity. However, SSDs, even for the same model, exhibit very different characteristics and degrade over time, leading to severe disk under-utilization. In this work, we present Asymmetric-RAID (Asym-RAID), a novel RAID architecture that optimizes system performance and storage utilization by exploiting heterogeneity from a larger SSD pool. Asym-RAID asymmetrically distributes data across the array to fully utilize the capacity of each SSD. To improve performance, Asym-RAID differentially exports the address space of each data stripe to the host, allowing for performance-optimized data placement. We outline the necessary changes in the storage stack for building an asymmetric RAID system and highlight its benefits. 

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2. eZNS: Elastic Zoned Namespace for Enhanced Performance Isolation and Device Utilization

   https://doi.org/10.1145/3653716  

   Min, Jaehong; Zhao, Chenxingyu; Liu, Ming; Krishnamurthy, Arvind (August 2024, ACM Transactions on Storage)

   Emerging Zoned Namespace (ZNS) SSDs, providing the coarse-grained zone abstraction, hold the potential to significantly enhance the cost efficiency of future storage infrastructure and mitigate performance unpredictability. However, existing ZNS SSDs have a static zoned interface, making them in-adaptable to workload runtime behavior, unscalable to underlying hardware capabilities, and interfering with co-located zones. Applications either under-provision the zone resources yielding unsatisfied throughput, create over-provisioned zones and incur costs, or experience unexpected I/O latencies. We propose eZNS, an elastic-ZNS interface that exposes an adaptive zone with predictable characteristics. eZNS comprises two major components: a zone arbiter that manages zone allocation and active resources on the control plane, and a hierarchical I/O scheduler with read congestion control and write admission control on the data plane. Together, eZNS enables the transparent use of a ZNS SSD and closes the gap between application requirements and zone interface properties. Our evaluations over RocksDB demonstrate that eZNS outperforms a static zoned interface by 17.7% and 80.3% in throughput and tail latency, respectively, at most. 

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3. Breathing New Life into an Old Tree: Resolving Logging Dilemma of B ⁺ -tree on Modern Computational Storage Drives

   https://doi.org/10.14778/3626292.3626297  

   Huang, Kecheng; Shen, Zhaoyan; Shao, Zili; Zhang, Tong; Chen, Feng (October 2023, Proceedings of the VLDB Endowment)

   Having dominated databases and various data management systems for decades,B⁺-tree is infamously subject to a logging dilemma: One could improveB⁺-tree speed performance by equipping it with a larger log, which nevertheless will degrade its crash recovery speed. Such a logging dilemma is particularly prominent in the presence of modern workloads that involve intensive small writes. In this paper, we propose a novel solution, calledper-page loggingbasedB⁺-tree, which leverages the emerging computational storage drive (CSD) with built-in transparent compression to fundamentally resolve the logging dilemma. Our key idea is to divide the large single log into many small (e.g., 4KB), highly compressibleper-page logs, each being statically bounded with aB⁺-tree page. All per-page logs together form a very large over-provisioned log space forB⁺-tree to improve its operational speed performance. Meanwhile, during crash recovery,B⁺-tree does not need to scan any per-page logs, leading to a recovery latency independent from the total log size. We have developed and open-sourced a fully functional prototype. Our evaluation results show that, under small-write intensive workloads, our design solution can improveB⁺-tree operational throughput by up to 625.6% and maintain a crash recovery time of as low as 19.2 ms, while incurring a minimal storage overhead of only 0.5-1.6%. 

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4. Reprogramming 3D TLC Flash Memory based Solid State Drives

   https://doi.org/10.1145/3487064  

   Gao, Congming; Ye, Min; Xue, Chun Jason; Zhang, Youtao; Shi, Liang; Shu, Jiwu; Yang, Jun (February 2022, ACM Transactions on Storage)

   NAND flash memory-based SSDs have been widely adopted. The scaling of SSD has evolved from plannar (2D) to 3D stacking. For reliability and other reasons, the technology node in 3D NAND SSD is larger than in 2D, but data density can be increased via increasing bit-per-cell. In this work, we develop a novel reprogramming scheme for TLCs in 3D NAND SSD, such that a cell can be programmed and reprogrammed several times before it is erased. Such reprogramming can improve the endurance of a cell and the speed of programming, and increase the amount of bits written in a cell per program/erase cycle, i.e., effective capacity. Our work is the first to perform a real 3D NAND SSD test to validate the feasibility of the reprogram operation. From the collected data, we derive the restrictions of performing reprogramming due to reliability challenges. Furthermore, a reprogrammable SSD (ReSSD) is designed to structure reprogram operations. ReSSD is evaluated in a case study in RAID 5 system (RSS-RAID). Experimental results show that RSS-RAID can improve the endurance by 35.7%, boost write performance by 15.9%, and increase effective capacity by 7.71%, with negligible overhead compared with conventional 3D SSD-based RAID 5 system. 

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5. Storage Abstractions for SSDs: The Past, Present, and Future

   https://doi.org/10.1145/3708992  

   Zhang, Xiangqun; Bhimani, Janki; Pei, Shuyi; Lee, Eunji; Lee, Sungjin; Seong, Yoon Jae; Kim, Eui Jin; Choi, Changho; Nam, Eyee Hyun; Choi, Jongmoo; et al (February 2025, ACM Transactions on Storage)

   This article traces the evolution of SSD (solid-state drive) interfaces, examining the transition from the block storage paradigm inherited from hard disk drives to SSD-specific standards customized to flash memory. Early SSDs conformed to the block abstraction for compatibility with the existing software storage stack, but studies and deployments show that this limits the performance potential for SSDs. As a result, new SSD-specific interface standards emerged to not only capitalize on the low latency and abundant internal parallelism of SSDs, but also include new command sets that diverge from the longstanding block abstraction. We first describe flash memory technology in the context of the block storage abstraction and the components within an SSD that provide the block storage illusion. We then describe the genealogy and relationships among academic research and industry standardization efforts for SSDs, along with some of their rise and fall in popularity. We classify these works into four evolving branches: (1) extending block abstraction with host-SSD hints/directives; (2) enhancing host-level control over SSDs; (3) offloading host-level management to SSDs; and (4) making SSDs byte-addressable. By dissecting these trajectories, the article also sheds light on the emerging challenges and opportunities, providing a roadmap for future research and development in SSD technologies. 

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