  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
which are in the same rack is much lower than between nodes which are in
different racks.
  In this paper, a new model, where the storage nodes are placed in two racks,
is proposed and analyzed. Moreover, the two-rack model is generalized to any
number of racks. In this model, the storage nodes have different repair costs
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
ones given for 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.
