  In distributed storage systems that use coding, the issue of minimizing the
communication required to rebuild a storage node after a failure arises. We
consider the problem of repairing an erased node in a distributed storage
system that uses an EVENODD code. EVENODD codes are maximum distance separable
(MDS) array codes that are used to protect against erasures, and only require
XOR operations for encoding and decoding. We show that when there are two
redundancy nodes, to rebuild one erased systematic node, only 3/4 of the
information needs to be transmitted. Interestingly, in many cases, the required
disk I/O is also minimized.
