  Caching is frequently used by Internet Service Providers as a viable
technique to reduce the latency perceived by end users, while jointly
offloading network traffic. While the cache hit-ratio is generally considered
in the literature as the dominant performance metric for such type of systems,
in this paper we argue that a critical missing piece has so far been neglected.
Adopting a radically different perspective, in this paper we explicitly account
for the cost of content retrieval, i.e. the cost associated to the external
bandwidth needed by an ISP to retrieve the contents requested by its customers.
Interestingly, we discover that classical cache provisioning techniques that
maximize cache efficiency (i.e., the hit-ratio), lead to suboptimal solutions
with higher overall cost. To show this mismatch, we propose two optimization
models that either minimize the overall costs or maximize the hit-ratio,
jointly providing cache sizing, object placement and path selection. We
formulate a polynomial-time greedy algorithm to solve the two problems and
analytically prove its optimality. We provide numerical results and show that
significant cost savings are attainable via a cost-aware design.
