## 6 Evaluation
### Experimental Setup  
We consider a system consisting of multiple RSU nodes and vehicle devices, which are randomly distributed within a specified area. In this system, RSU nodes generate and store blockchain data, while vehicles access these blocks based on their individual demands. Given the heterogeneity of available storage resources among RSUs at a given time, we assume that the available storage capacity of each RSU node follows a normal distribution within the range of \([200,800]\) MB. The communication cost between RSUs depends on their positions and the network topology. RSU nodes can communicate either directly or via multi-hop transmission. Meanwhile, vehicles move within the coverage area of RSUs, request block data, and eventually leave the RSU coverage area after a certain period. The dwell time of a vehicle within an RSU's coverage area follows a uniform distribution within the range \([0,t^{d}]\).  
Hyperledger Fabric [26] is a permissioned blockchain platform that has been widely adopted in IoT and IoV applications [27; 28] due to its robust security features and flexible architecture. We utilize the Hyperledger Fabric consortium blockchain platform to construct the blockchain network, adopting its default parameter constraints. Specifically, the block size is limited to 10 MB, each block can contain a maximum of 10 transactions, and the maximum data size per transaction is 512 KB. We assume that the blockchain network has been running for a certain period, and the existing block sizes follow a uniform distribution within the range \([5,10]\) MB. For each newly created block, its initial query frequency is randomly assigned as \(F_{0}\), while its block age \(A_{k}\) starts from 0 and gradually increases as new blocks are generated.  
For the proposed RSU clustering scheme, we choose the following clustering algorithms for comparison based on VSPGBAA as the allocation algorithm:  
* Random Clustering: RSU nodes are clustered randomly with their neighboring nodes without considering vehicle access preferences until each cluster satisfies the blockchain storage capacity constraint.
* Single-Cluster Scheme: No preprocessing is applied to RSU nodes. Instead, all RSUs are treated as a single cluster, and block allocation is performed directly within this global cluster. In the subsequent block allocation phase, all methods adopt the VSPGBAA algorithm proposed in this paper.  
For the proposed intra-cluster block allocation scheme, we perform clustering based on the RAPSCA algorithm and select the following allocation algorithms for comparison:  
* Two-Step Greedy Algorithm (TSGA) [22]: This algorithm considers the impact of the allocation process on total access cost. For each allocation step, it pre-evaluates the change in total query cost after allocation and selects the decision that results in the greatest cost reduction. After each allocation, it iterates over all blocks and RSUs, updating the expected query cost for further decisions.
* Node First Algorithm [29]: This algorithm first selects the RSU that has the highest demand for each block to store its first backup copy. Then, for each RSU, it considers only local demand, sequentially storing the most required blocks until the available storage capacity is fully utilized.  
Random Allocation: This algorithm does not consider RSU block demand but ensures that each block has at least one backup to maintain blockchain ledger integrity within the cluster. After initial placement, blocks are sorted based on their demand, and randomly assigned to RSUs with available storage until all storage resources in the cluster are utilized.