Data reliability are drawn much concern in large-scale storage systems built from thousands of storage devices, which highly depends on many inter-dependent system parameters, such as the replica placement policies, number of stored objects and so on. Previous work has discussed the impacts of these system parameters on reliability roughly and separately, and seldom provided their optimal values, nor mentioned their optimal combination. This paper presents a new object-based-repairing analytic model. Based on analyzing this model in three popular replica placement policies, it figures out the ...