  In a distributed storage environment, where the data is placed in nodes
connected through a network, it is likely that one of these nodes fails. It is
known that the use of erasure coding improves the fault tolerance and minimizes
the redundancy added in distributed storage environments. The use of
regenerating codes not only make the most of the erasure coding improvements,
but also minimizes the amount of data needed to regenerate a failed node.
  In this paper, a new family of regenerating codes based on quasi-cyclic codes
is presented. Quasi-cyclic flexible minimum storage regenerating (QCFMSR) codes
are constructed and their existence is proved. Quasi-cyclic flexible
regenerating codes with minimum bandwidth constructed from a base QCFMSR code
are also provided. These codes not only achieve optimal MBR parameters in terms
of stored data and repair bandwidth, but also for an specific choice of the
parameters involved, they can be decreased under the optimal MBR point.
  Quasi-cyclic flexible regenerating codes are very interesting because of
their simplicity and low complexity. They allow exact repair-by-transfer in the
minimum bandwidth case and an exact pseudo repair-by-transfer in the MSR case,
where operations are needed only when a new node enters into the system
replacing a lost one.
