Peer-to-peer systems are foreseen as an efficient solution to achieve reliable data storage at low cost. To deal with common P2P problems such as peer failures or churn, such systems encode the user data into redundant fragments and distribute them among peers. The way they distribute it, known as placement policy, has a significant impact on their behavior and reliability. In this report, after a brief state-of-the-art of the technology used in P2P storage systems, we compare three different placement policies: two of them local, in which the data is stored in logical peer neighborhoods, and on of them global in which fragments are parted at random among the different peers. For each policy, we give either Markov Chain Models to efficiently compute the Mean Time To Data Loss (which is closely related to the probability to lose data) or approximations of this quantity under certain assumptions. We also attempt to give lower bounds on P2P storage systems introducing the BIG system, in which we consider information globally. We propose various ways to compute a bound on the probability to lose data, in relation with parameters such as the peer failure rate of the peer bandwidth.