  In distributed storage systems (DSSs), the optimal tradeoff between node
storage and repair bandwidth is an important issue for designing distributed
coding strategies to ensure large scale data reliability. The capacity of DSSs
is obtained as a function of node storage and repair bandwidth parameters,
characterizing the tradeoff. There are lots of works on DSSs with clusters
(racks) where the repair bandwidths from intra-cluster and cross-cluster are
differentiated. However, separate nodes are also prevalent in the realistic
DSSs, but the works on DSSs with clusters and separate nodes (CSN-DSSs) are
insufficient. In this paper, we formulate the capacity of CSN-DSSs with one
separate node for the first time where the bandwidth to repair a separate node
is of cross-cluster. Consequently, the optimal tradeoff between node storage
and repair bandwidth are derived and compared with cluster DSSs. A regenerating
code instance is constructed based on the tradeoff. Furthermore, the influence
of adding a separate node is analyzed and formulated theoretically. We prove
that when each cluster contains R nodes and any k nodes suffice to recover the
original file (MDS property), adding an extra separate node will keep the
capacity if R|k, and reduce the capacity otherwise.
