With the increasing development of SSD (Solid-State Drives) technology, SSD RAID (Redundant Arrays of Independent Disks) has been widely deployed in enterprise data centers. However, the inherent write endurance issue of SSD seriously affects the reliability of the array. Meanwhile, compared with conventional HDD-based RAID, SSD RAID exhibits very different failure characteristics, such as correlated failure (Balakrishnan et al., 2010) under RAID-5. In this paper, we present a Hybrid High reliability RAID architecture, named H${}^2$-RAID, by equipping each SSD with an extra HDD as the backup to improve the reliability of SSD RAID. Considering the relatively longer write latency of HDD, in H${}^2$-RAID, we first propose an HDD-aware backup mechanism to smartly aggregate random writes into sequential writes to decrease performance degradation. In addition, to cope with the scenarios of SSD failure, an HDD-aware reconstruction method is further proposed to guarantee the reliability and the online transaction processing performance. We build a novel markov process-based mathematical model to analyze the reliability of different architectures, and the theoretical results prove the reliability of H${}^2$-RAID is much higher than that of traditional SSD RAID. To more accurately evaluate the performance influence of HDD on H${}^2$-RAID, we develop a simulator based on Disksim and the experimental results show H${}^2$-RAID signicantly increases the reliability compared with SSD array (under RAID-5) while with little performance loss on average.

随着SSD（固态硬盘）技术的日益发展，SSD RAID（独立磁盘冗余阵列）已在企业数据中心广泛部署。然而，SSD固有的耐写问题严重影响了阵列的可靠性。同时，与传统的基于 HDD 的 RAID 相比，SSD RAID 表现出截然不同的故障特征，例如 RAID-5 下的相关故障（Balakrishnan 等，2010）。在本文中，我们提出了一种混合高可靠性 RAID 架构，称为 H${}^2$-RAID，通过为每个SSD配备额外的HDD作为备份，提高SSD RAID的可靠性。考虑到HDD的写入延迟相对较长，在H${}^2$-RAID，我们首先提出了一种HDD感知备份机制，可以智能地将随机写入聚合为顺序写入，以减少性能下降。此外，为了应对SSD故障的场景，进一步提出了HDD感知重建方法，以保证可靠性和在线事务处理性能。我们建立了一种新颖的基于马尔可夫过程的数学模型来分析不同架构的可靠性，理论结果证明了H的可靠性${}^2$-RAID远高于传统SSD RAID。为了更准确地评估HDD对H的性能影响${}^2$-RAID，我们基于Disksim开发了一个模拟器，实验结果显示H${}^2$-与 SSD 阵列（RAID-5 下）相比，RAID 显着提高了可靠性，同时平均性能损失很小。