  In a realistic distributed storage environment, storage nodes are usually
placed in racks, a metallic support designed to accommodate electronic
equipment. It is known that the communication (bandwidth) cost between nodes
within a rack is much lower than the communication (bandwidth) cost between
nodes within different racks.
  In this paper, a new model, where the storage nodes are placed in two racks,
is proposed and analyzed. In this model, the storage nodes have different
repair costs to repair a node depending on the rack where they are placed. A
threshold function, which minimizes the amount of stored data per node and the
bandwidth needed to regenerate a failed node, is shown. This threshold function
generalizes the threshold function from previous distributed storage models.
The tradeoff curve obtained from this threshold function is compared with the
ones obtained from the previous models, and it is shown that this new model
outperforms the previous ones in terms of repair cost.
