Virtual world has the potential to become a future global electronic market, integrating many isolated markets in many areas. To achieve this goal, future virtual world is required to be persistent, which means that a virtual world together with its accumulated content shall exist forever regardless of the dynamic changes of its virtual world users and virtual world owners. This paper addresses the content-level persistence issue by proposing a persistence framework which consists of three elements: existence, availability, and sufficiency. Following the framework, a decentralized redundancy model is then proposed, which is immune from the possible collapse of virtual world. The key problem in the model is the selection of a redundancy level, which is based on cost optimization. Furthermore, the replica failure timeout mechanism is introduced to improve the model. The simulation results show that the redundancy model helps improve content availability and minimizes cost.