The role of the acquisition interval for the cost-effective use of public and private storage for the cloud infrastructure is the focus of this paper. It introduces a storage cost model to compare the cost of private and public storage over a period of time. The authors provide mathematical proof for five propositions, which form the basis of the model:

(1) The cost of private storage increases as the length of the acquisition interval (for additional storage capacity) increases.

(2) For a given cost function, the cost difference between public and private storage decreases as the length of the acquisition interval increases.

(3) If the demand for storage capacity grows exponentially with time, the cost difference of public and private storage decreases as the acquisition interval length increases.

(4) If the demand for storage capacity grows linearly with time, the cost difference of public and private storage increases as the acquisition interval length increases.

(5) If the demand for storage capacity grows logarithmic with time, the cost difference of public and private storage increases as the acquisition interval length increases.

The authors illustrate and discuss the approach through numerical examples, acknowledge a need for further work, and hint at possible directions. This well-written paper gives numerous references, and is a good foundational work in the area.