Virtual world has the potential to become a future global electronic marketplace, integrating many isolated markets in many areas. To achieve this goal, future virtual world is required to be persistent, implying that a virtual world together with its accumulated content shall exist forever regardless of dynamic changes of its users and owners. Unfortunately, existing virtual worlds, owning by some entities, are not immune from death due to business entity failure. To provide a persistent virtual world, a decentralized architecture is explored, which is constructed on user contributed devices. However, there are many challenges to realize a decentralized virtual world. One important issue is user cooperation in reliable content storage. The devices contributed by users may not be reliable for maintaining all user contents, but users do not have the incentive to provide reliable devices for others. This paper addresses the issue by two steps. First, an indicator, called replica group reliability, is provided to users, which is based on the proposed replicability index. Based on the indicator, users can learn the reliability of their content storage. Then, a new user incentive mechanism, called equity-based node allocation strategy, is proposed to promote user cooperation to collectively maintain reliable content storage. A decentralized algorithm implementing the strategy is designed and the evaluation results show its effectiveness and efficiency.