Role of acquisition intervals in private and public cloud storage costs

• The paper introduces an analytical model for private and public storage costs.
• We study the role of acquisition interval on the cost-efficiency of cloud storage.
• The shorter the acquisition interval, the less expensive the private storage is.
• Numerical examples are presented on storage costs of a back-up and archiving system.

The volume of worldwide digital content has increased nine-fold within the last five years, and this immense growth is predicted to continue in the foreseeable future to reach 8 ZB by 2015. Traditionally, organizations proactively have built and managed their private storage facilities to cope with the growing demand for storage capacity. Recently, many organizations have instead welcomed the alternative of outsourcing their storage needs to the providers of public cloud storage services due to the proliferation of public cloud infrastructure offerings. The comparative cost-efficiency of these two alternatives depends on a number of factors, such as the prices of the public and private storage, the charging and the storage acquisition intervals, and the predictability of the demand for storage. In this paper, we study the relationship between the cost-efficiency of the private vs. public storage and the acquisition interval at which the organization re-assesses its storage needs and acquires additional private storage. The analysis in the paper suggests that for commonly encountered exponential growth of storage demand, shorter acquisition intervals increase the likelihood of less expensive private storage solutions compared with public cloud infrastructure. This phenomenon is also numerically illustrated in the paper using the storage needs encountered by a university back-up and archiving service as an example. Because the acquisition interval is determined by the organization's ability to foresee the growth of storage demand, via provisioning schedules of storage equipment providers, and internal practices of the organization, among other factors, organizations that own a private storage solution may want to control some of these factors to attain a shorter acquisition interval and thus make the private storage (more) cost-efficient.