## 6 Evaluation
### Performance Evaluation
#### 6.2.2 Intra-Cluster Block Allocation Performance Evaluation  
To evaluate the effectiveness of the VSPGBAA algorithm, the experiments compare its storage hit rate, total access cost within the cluster, and algorithm runtime under different numbers of RSU nodes, varying vehicle counts, and different blockchain lengths. The baseline algorithms are introduced in the experiment setup in Section 6.1. * Impact of the number of RSU nodes on Intra-Cluster Block Allocation: As shown in Figure 5(a)-c, the system is configured with a fixed block count of 300, a fixed vehicle-to-RSU ratio of 9:1, and a fixed number of three clusters, while the number of RSU nodes varies from 20 to 100. As illustrated in Figure 5(a),b, when the number of RSU nodes within a cluster is small, the storage hit rate and total access cost of VSPGBAA are close to those of TSGA. However, as the number of RSUs within the cluster increases, the storage hit rate of VSPGBAA remains stable but is slightly lower than that of TSGA. Specifically, compared to TSGA, VSPGBAA incurs 16.4% higher access cost, and achieves a 36% lower cache hit rate, but significantly improves efficiency with a 16\(\times\) shorter runtime. Meanwhile, as shown in Figure 5(c), the running time of TSGA increases sharply as the number of RSUs in the cluster grows, greatly exceeding that of VSPGBAA, NodeFirst, and the random allocation strategy. This is because TSGA globally updates the entire system state for each block allocation, whereas VSPGBAA only updates the local neighborhood affected by the allocation. As a result, VSPGBAA reduces computational overhead while maintaining competitive performance, making it adaptable to different cluster sizes. Furthermore, compared to NodeFirst, VSPGBAA reduces total access cost by 26.9%, improves the cache hit rate by 34%, but requires 19.5% more runtime. This trade-off highlights VSPGBAA's ability to balance efficiency and performance, demonstrating its potential in large-scale deployments. * Impact of the number of vehicles on Intra-Cluster Block Allocation: As shown in Figure 6(a)-c, the system consists of three clusters, 20 RSUs, and 300 blocks to be allocated, while the total number of vehicles varies from 20 to 180. As illustrated in  
Figure 5: Results of clustering algorithms with the number of clusters. Figure 6: Results of block allocation algorithms with the number of RSU nodes. Figure 7a,b, as the number of vehicles increases, the volume of access requests within the cluster grows, leading to a linear increase in total access cost, while the storage hit rate remains relatively stable. The storage hit rate of VSPGBAA is slightly lower than that of TSGA but significantly outperforms the random and NodeFirst allocation strategies. Specifically, compared to TSGA, VSPGBAA incurs a 32.9% higher access cost, and achieves a 10.6% lower cache hit rate, but compensates with a 6.7\(\times\) shorter runtime, demonstrating a strong trade-off between efficiency and performance. Meanwhile, as shown in Figure 7c, the runtime of TSGA increases significantly with the number of vehicles, greatly exceeding that of VSPGBAA and NodeFirst. Additionally, compared to NodeFirst, VSPGBAA reduces total access cost by 61.5%, and improves the cache hit rate by 20.2%, but requires 25% more runtime. This highlights VSPGBAA's ability to handle high-traffic vehicular environments effectively, striking a balance between computational efficiency and system performance. * Impact of the number of blocks on the RSU Clustering Process: As shown in Figure 8a-c, the system consists of three clusters, 60 RSU nodes, and 540 vehicles, while the number of blocks to be allocated varies from 200 to 600. As illustrated in Figure 8a,b, as the number of blocks increases, the storage hit rate declines, while the total access cost gradually rises. This is because the increasing amount of block data reduces the available storage space for redundant copies within the cluster, thereby limiting the effectiveness of access cost optimization.