  Private information retrieval scheme for coded data storage is considered in
this paper. We focus on the case where the size of each data record is large
and hence only the download cost (but not the upload cost for transmitting
retrieval queries) is of interest. We prove that the tradeoff between storage
cost and retrieval/download cost depends on the number of data records in the
system. We also propose a fairly general class of linear storage codes and
retrieval schemes and derive conditions under which our retrieval schemes are
error-free and private. Tradeoffs between the storage cost and retrieval costs
are also obtained. Finally, we consider special cases when the underlying
storage code is based on an MDS code. Using our proposed method, we show that a
randomly generated retrieval scheme is indeed very likely to be private and
error-free.
