Reliability evaluation of tape library systems

Abstract

Magnetic tape is a digital data storage technology that has evolved continuously over the last seven decades. It provides a cost-effective way to retain the rapidly increasing volumes of data being created in recent years. The low cost per terabyte combined with tape’s low energy consumption make it an appealing option for storing infrequently accessed data and has resulted in a resurgence in use of the technology. Power and operational failures may damage tapes and lead to data loss. To protect stored data against loss and achieve high data reliability, an erasure coding scheme is employed. A theoretical model capturing the effect of tape failures and latent errors on system reliability is developed. Closed-form expressions are derived for the Mean Time to Data Loss ($\text{MTTDL}$) and the Expected Annual Fraction of Effective Data Loss ($\text{EAFEDL}$) reliability metric, which assesses losses at the file, object, or block, level. The results obtained demonstrate that, for realistic values of bit error rates, reliability is affected by the presence of latent errors. The effect of system parameters on reliability is assessed by conducting a sensitivity evaluation. The reliability improvement achieved by employing erasure coding schemes with increased capability is demonstrated. The theoretical results derived can be used to dimension and provision tape libraries to provide desired levels of data durability.