Permissioned blockchain has emerged as an effective solution to IoT data security challenges. With its decentralization, immutability, and transparency, integrating blockchain with IoT enhances security and improves data availability. Due to the performance limitations of underlying blockchains in handling massive IoT data, existing IoT blockchains typically adopt a two-layer architecture. In this architecture, the actual data is stored off-chain (e.g., in cloud servers or cloud storage) while only lightweight proofs are maintained on-chain. However, this approach faces several challenges. First, off-chain storage introduces vulnerability issues such as data loss and leaks. Second, synchronization between the off-chain data and the on-chain proofs becomes a performance bottleneck. To address these limitations, we propose DC-Storage, a fast permissioned IoT blockchain with a dual-chain architecture. We identify index construction as the primary performance bottleneck and decouple the index chain from the data chain to enable fast data chain construction with asynchronous indexing. Since transactions are considered committed after modifying the data chain, this significantly improves system throughput. Furthermore, we enable parallel index construction, substantially accelerating the indexing process. Experimental results show that DC-Storage outperforms its baseline with \(5.03\times \) higher throughput.