## 1 Introduction  
With the rapid development of the Internet of Things (IoT) technology, a wide variety of smart devices are becoming interconnected. Internet of Vehicles (IoV), a key IoT application in the vehicular domain, integrates vehicles, roadside units (RSUs), and cloud platforms to form a collaborative network for resource sharing. This integration significantly enhances data exchange efficiency and improves the intelligence level of transportation systems. Building upon the foundation of IoV, Social Internet of Vehicles (SloV) introduces social relationships into vehicular networks, enabling vehicles to interact based on trust relationships and shared interests. This transformation allows vehicles to evolve from passive data transmission nodes into intelligent entities that actively construct and manage social relationship networks. Consequently, SloV further promotes internal information exchange and resource sharing within the network [1, 2, 3, 4]. Blockchain offers a promising solution for distributed reputation management in SloV through decentralization, data immutability, and smart contract automation [5, 6, 7]. In blockchain-enabled SIoV, blockchain serves as a trusted infrastructure that records vehicle interactions through a distributed ledger and consensus mechanisms, ensuring data authenticity and reliability. However, the integration of blockchain introduces additional storage burdens to SIoV systems. Due to the append-only nature and multiple-replica storage mechanism of blockchain, its storage demand continuously increases over time [8]. For RSU nodes with limited and heterogeneous storage resources, this ever-growing demand may become a performance bottleneck, thereby impacting the scalability and operational efficiency of the entire SIoV system [9; 10; 11]. A feasible approach to addressing storage performance challenges in SIoV is collaborative storage. This method enables blockchain data to be distributed across multiple storage nodes, with each node storing only a portion of the data. When full data access is required, nodes can efficiently retrieve missing blocks through an inter-node data exchange mechanism, thereby reducing the storage burden on individual nodes and enhancing overall resource utilization. Several studies have explored collaborative storage in blockchain, such as leveraging the InterPlanetary File System (IPFS) [12] or employing consistent hashing-based encoding schemes [13] for distributed storage. However, these studies primarily focus on storage balance and fairness, overlooking access costs in the SIoV context. Therefore, designing an optimized collaborative storage scheme tailored for RSUs in blockchain networks is essential to improving the performance and efficiency of SIoV systems. There are still several challenges in optimizing collaborative storage for RSUs in a blockchain-enabled SIoV system:  
* In traditional IoV systems, RSUs operate independently without clustering or collaboration. In blockchain-enabled SIoV, RSUs must collaboratively maintain blockchain integrity despite limited storage resources. The full-node storage model imposes high demands, restricting resource-constrained RSUs from participating in consensus, thereby limiting network scalability. Existing clustering methods, based on geographical location or network topology, overlook vehicles' block access patterns. Thus, an RSU clustering strategy that integrates blockchain data characteristics while addressing storage constraints is essential for enhancing efficiency and performance. * In a collaborative storage cluster, block allocation among RSUs directly impacts vehicle access latency and service quality. Existing methods, relying on general content popularity, fail to account for vehicle mobility and block access correlations, leading to higher access costs. Therefore, developing an RSU cluster-based storage allocation scheme tailored to blockchain scenarios is crucial for minimizing vehicle access costs and enhancing system performance.