The paper proposes an approach to solving the problem of optimal placement of data array replicas in a distributed system. It presents a formal model of a distributed system with unreliable communication channels. A system is represented as a graph with coloured arcs. The formulation of the problem of finding the optimal placement of replicas is given. As a criterion for optimization, the minimum response time to a data request is considered. The task solution heuristic algorithm is also proposed in the paper.