ROS: a Rack-Based Optical Storage System with Inline Accessibility for Long-Term Data Preservation

ROS: A Rack-based Optical Storage System with Inline Accessibility for Long-Term Data Preservation Wenrui Yan Jie Yao ∗ Qiang Cao † Hong Jiang Changsheng Xie Dept. of CSE University of Texas at Arlington Wuhan National Laboratory for Optoelectronics, [email protected] Key Laboratory of Information Storage System, Ministry of Education of China School of Computer Science and Technology, Huazhong University of Science and Technology {wenrui yan, jackyao, caoqiang, cs xie}@hust.edu.cn Abstract prototype PB-level ROS system. The results show that ROS The combination of the explosive growth in digital data and stacked on Samba and FUSE can provide almost 323MB/s the need to preserve much of this data in the long term read and 236MB/s write throughput, about 53ms file write has made it an imperative to find a more cost-effective way and 15ms read latency via 10GbE network for external users, than HDD arrays and more easily accessible way than tape exhibiting its inline accessibility. Besides, ROS is able to ef- libraries to store massive amounts of data. While modern fectively hide and virtualize internal complex operational be- optical discs are capable of guaranteeing more than 50- haviors and be easily deployable in datacenters. year data preservation without migration, individual optical CCS Concepts • Information systems → Cloud based disks’ lack of the performance and capacity relative to HDDs storage or tapes has significantly limited their use in datacenters. This paper presents a Rack-scale Optical disc library Sys- tem, or ROS in short, that provides a PB-level total capacity 1. Introduction and inline accessibility on thousands of optical discs built It has been considered highly desirable for much the digital within a 42U Rack. A rotatable roller and robotic arm sep- data generated from various sources such as scientific re- arating and fetching the discs are designed to improve disc search, social and commercial activities to be preserved for placement density and simplify the mechanical structure. A more than 50 years or longer, if not forever [9, 23, 26, 28]. In hierarchical storage system based on SSD, hard disks and addition, since emerging big data analytics technologies en- optical discs are presented to hide the delay of mechanical able to further unlock the value of data, an increasing amount operation. On the other hand, an optical library file system of data that used to be discarded on a daily basis in the past is proposed to schedule mechanical operation and organize are now considered target for long-term preservation once data on the tiered storage with a POSIX user interface to pro- the storage systems become more cost-effective. More im- vide an illusion of inline data accessibility. We evaluate ROS portantly, increasing analytics applications that depend on on a few key performance metrics including operation de- mining historical data require inline accessibility to these lays of the mechanical structure and software overhead in a long-term preserved data in order to produce timely results. In other words, these data should be conveniently accessed ∗ Corresponding author: [email protected] by big data analytics applications in a manner similar to ac- † Corresponding author: [email protected] cessing databases or filesystems, without the cumbersome interventions of conventional archival or backup systems. Unfortunately, reliably preserving data for the long term remains a significant challenge [10]. Mainstream online Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and storage media such as solid state drive (SSD) and hard disks the full citation on the first page. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, or republish, to post on servers or to redistribute to lists, requires (HDD) have limited lifecycles of up to 5 years in which prior specific permission and/or a fee. Request permissions from [email protected]. data reliably stored [3, 15]. Offline storage media like mag- EuroSys ’17, April 23-26, 2017, Belgrade, Serbia c 2017 ACM. ISBN 978-1-4503-4938-3/17/04. $15.00 netic tapes only guarantee 10-year data preservation [19]. DOI: http://dx.doi.org/10.1145/3064176.3064207 For longer-term data preservation, data migration from old 161 media to new media is inevitable, increasing risks of data Architecture: ROS combines long-term data preserva- loss and complexity of maintenance[14]. Additionally, these tion of optical discs with fast access performance of hard three typical mainstream storage media demand strict envi- disks and SSDs to consolidate the best characteristics of the ronmental conditions such as constant temperature and hu- three technologies, constituting an externally transparent and midity, thus consuming extra amount of energy. internally tiered storage architecture based on SSDs and/or Optical discs are shown to be able to reliably store HDDs, referred indistinguishably as disks hereafter unless data in excess of 50 years without specific environmental noted otherwise, optical drives, and discs in rollers. All the requirements[11], far outperforming hard disks and tapes disks and drives are connected to a server-based controller in terms of reliable life expectancy and management cost. accessible by external clients via high-speed network such Since each optical disc is made of a piece of plastic and as 10GbE and Infiniband, making ROS a network attached multiple thin layers of coating films at sereval ums [18], storage. The disks are used as write buffer and read cache. the material and manufacturing costs of optical discs are All optical drives can be accessed in parallel to ensure high very low. Furthermore, considering the data migration and sustained throughput. This design guarantees that all write maintenance cost, the TCO (Total Cost of Ownership) of requests can be completely buffered and most read requests optical-discs based storage is significantly lower than that of can hit in disks to effectively hide the tens-of-seconds la- hard-disks or tapes based storage for long-term data preser- tency of the mechanical operations. vation. Nevertheless, there are three fundamental limitations Software: ROS develops an optical library file system of optical discs that greatly hamper their use in any storage (OLFS) to orchestrate the interplay and interaction among systems, much less a stand-alone optical storage. First, the software, hardware, and mechanical subsystems. It offers a capacity of a single optical disc is still far less than that of general and programmable POSIX interface to realize inline a hard disk or tape. Second, the maximal access throughput accessibility for external applications, as well as effectively for an optical disc is up to 40MB/s, far lower than that for hide the long mechanical delays. OLFS can automatically a hard disk. Third, the writing pattern for optical discs is access and manage PB-level data across tens of thousands preferred to be write-all-once, where all prepared data are of discs without manually partitioning datasets across phys- burned onto discs once. ical discs and burning discs. OLFS maintains a global file In order to overcome these limitations of optical discs, view and strategically partitions all files into Universal Disc we present an optical library called ROS, which stands for a Format(UDF)[1] disc images on disks or discs. The global Rack-based Optical Storage system for long-term data stor- view is built on a fast and small mirroring volume while age with inline accessibility. ROS contains thousands of op- all file data and their abstract path are ultimately stored in tical discs to improve the overall storage capacity and tens disc images. ROS can further asynchronously generate par- of optical drives to enhance the sustained access through- ity disc images based on a set of prepared data disc images, put. More importantly, ROS is inline accessible and can enhancing data reliability at the system level. be deployed easily. It can be conveniently integrated into Evaluation: We ran a series of experiments to evaluate rack-scale datacenters without the introduction of an ex- ROS performance. The latency of critical operations, such tra backup or archival system. At the heart of ROS is a as accessing disks and discs, and mechanical behaviors, can novel system-level co-design of mechanical-electrical and be further analyzed. The experimental results also illustrate hardware-software that provides general and virtualized that ROS can effectively and transparently respond to file POSIX storage interfaces for transparent, inline access of requests through the POSIX interface at an acceptable per- the long-term preserved data from the user space. The main formance for inline accessing long-term preserved data[2]. contributions of ROS are summarized as follows. The rest of the paper is organized as follows. Section 2 Mechanical subsystem: To maximize the overall capac- provides the necessary background and analysis to motivate ity, ROS designs a high-precision electromechanical struc- our research on optical storage for long-term preserved data. ture to systematically organize up to 12240 optical discs, 48 The ROS mechanical and hardware designs are detailed in drives and a server-level controller in a 42U rack. This de- Section 3. Section 4 presents the design of the optical library sign offers a highly compact and relatively simple mechan- file system OLFS. In Section 5, we evaluate the performance ical structure. Each 6120 discs are placed in one of the two and functionalities of ROS. Finally, Section 6 and Section 7 rotatable rollers, each with 510 trays (of 12 discs each) that respectively discuss related work and conclude the paper.

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