An Exploratory Study on Software-Defined Data Center Hard Disk Drives

This article presents a design framework aiming to reduce mass data storage cost in data centers. Its underlying principle is simple: Assume one may noticeably reduce the HDD manufacturing cost by significantly (i.e., at least several orders of magnitude) relaxing raw HDD reliability, which ensures the eventual data storage integrity via low-cost system-level redundancy. This is called system-assisted HDD bit cost reduction. To better utilize both capacity and random IOPS of HDDs, it is desirable to mix data with complementary requirements on capacity and random IOPS in each HDD. Nevertheless, different capacity and random IOPS requirements may demand different raw HDD reliability vs. bit cost trade-offs and hence different forms of system-assisted bit cost reduction. This article presents a software-centric design framework to realize data-adaptive system-assisted bit cost reduction for data center HDDs. Implementation is solely handled by the filesystem and demands only minor change of the error correction coding (ECC) module inside HDDs. Hence, it is completely transparent to all the other components in the software stack (e.g., applications, OS kernel, and drivers) and keeps fundamental HDD design practice (e.g., firmware, media, head, and servo) intact. We carried out analysis and experiments to evaluate its implementation feasibility and effectiveness. We integrated the design techniques into ext4 to further quantitatively measure its impact on system speed performance.