  Distributed storage systems that deploy erasure codes can provide better
features such as lower storage overhead and higher data reliability. In this
paper, we focus on fractional repetition (FR) codes, which are a class of
storage codes characterized by the features of uncoded exact repair and minimum
repair bandwidth. We study the duality of FR codes, and investigate the
relationship between the supported file size of an FR code and its dual code.
Based on the established relationship, we derive an improved dual bound on the
supported file size of FR codes. We further show that FR codes constructed from
$t$-designs are optimal when the size of the stored file is sufficiently large.
Moreover, we present the tensor product technique for combining FR codes, and
elaborate on the file size hierarchy of resulting codes.
