ROS: A Rack-based Optical Storage System with Inline Accessibility for Long-Term Data Preservation.

The combination of the explosive growth in digital data and the need to preserve much of this data in the long term has made it an imperative to find a more cost-effective way than HDD arrays and more easily accessible way than tape libraries to store massive amounts of data. While modern optical discs are capable of guaranteeing more than 50-year data preservation without migration, individual optical disks' lack of the performance and capacity relative to HDDs or tapes has significantly limited their use in datacenters. This paper presents a Rack-scale Optical disc library System, or ROS in short, that provides a PB-level total capacity and inline accessibility on thousands of optical discs built within a 42U Rack. A rotatable roller and robotic arm separating and fetching the discs are designed to improve disc placement density and simplify the mechanical structure. A hierarchical storage system based on SSD, hard disks and optical discs are presented to hide the delay of mechanical operation. On the other hand, an optical library file system is proposed to schedule mechanical operation and organize data on the tiered storage with a POSIX user interface to provide an illusion of inline data accessibility. We evaluate ROS on a few key performance metrics including operation delays of the mechanical structure and software overhead in a prototype PB-level ROS system. The results show that ROS stacked on Samba and FUSE can provide almost 323MB/s read and 236MB/s write throughput, about 53ms file write and 15ms read latency via 10GbE network for external users, exhibiting its inline accessibility. Besides, ROS is able to effectively hide and virtualize internal complex operational behaviors and be easily deployable in datacenters.