  To recover simultaneous multiple failures in erasure coded storage systems,
Patrick Lee et al introduce concurrent repair based minimal storage
regenerating codes to reduce repair traffic. The architecture of this approach
is simpler and more practical than that of the cooperative mechanism in
non-fully distributed environment, hence this paper unifies such class of
regenerating codes as concurrent regenerating codes and further studies its
characteristics by analyzing cut-based information flow graph in the
multiple-node recovery model. We present a general storage-bandwidth tradeoff
and give closed-form expressions for the points on the curve, including
concurrent repair mechanism based on minimal bandwidth regenerating codes. We
show that the general concurrent regenerating codes can be constructed by
reforming the existing single-node regenerating codes or multiplenode
cooperative regenerating codes. Moreover, a connection to strong-MDS is also
analyzed. On the other respect, the application of RGC is hardly limited to
"repairing". It is of great significance for "scaling", a scenario where we
need to increase(decrease) nodes to upgrade(degrade) redundancy and
reliability. Thus, by clarifying the similarities and differences, we integrate
them into a unified model to adjust to the dynamic storage network.
