In a distributed storage system, private information retrieval (PIR) guarantees that a user retrieves one file from the system without revealing any information about the identity of its interested file to any individual server. In this paper, we investigate (N, K, M ) coded sever model of PIR, where each of M files is distributed to the N servers in the form of (N, K) maximum distance separable (MDS) code for some N > K and M > 1. As a result, we propose a new capacity-achieving (N, K, M ) coded linear PIR scheme such that it can be implemented with file length K(N−K) gcd(N,K) , which is much smaller than the previous best result K N gcd(N,K) M −1 . Notably, among all the capacity-achieving coded linear PIR schemes, we show that the file length is optimal if M > K gcd(N,K) − K N−K + 1, and within a multiplicative gap K gcd(N,K) of a lower bound on the minimum file length otherwise.
Index TermsPrivate information retrieval, distributed storage system, file length, coded servers, capacity-achieving. N M −1 −1 for any N and M .