## 1 Introduction  
Since the concept of peer-to-peer electronic cash transaction emerged in 2008 [1], blockchain technology and its underlying architecture have been applied in diverse areas including financial transactions, smart contracts, data management, storage, and communication [2]. Blockchain is a distributed ledger, maintained jointly by nodes over a distributed network. Nodes in the network maintain a local copy of the ledger by executing transactions authenticated by the consensus protocol. After more than 10 years of practical tests, blockchain technology has been recognized by many industries for the security of its transactions, data transparency, and the immutability of its transaction records. This has helped to enhance trust in direct electronic transactions without the need for trusted third-party intermediaries. However, the nodes in the blockchain network locally store multiple replicas of the entire blockchain, causing pressure on storage resources. Certain applications generate large quantities of transactional data, such as product traceability and scenarios from the Internet of Things (IoT), presenting challenges that cannot be adequately addressed by typical blockchain storage mechanisms [3]. Rapid growth of blockchain data creates huge storage pressure on the participating nodes, increasingly forcing nodes with limited storage resources to withdraw from the blockchain network, causing vulnerabilities in the stability of the system [4]. The expansion of blockchain storage capacity has become a major problem that limits the applicability and usability of blockchain technology [5]. Blockchain storage solutions that reduce infrastructure requirements are urgently needed. Sharding has been used for traditional database capacity expansion by splitting large data collections across multiple servers [6]. The Elastico system [7], proposed in 2016, is an applied sharding protocol for improving blockchain efficiency [8; 9; 10]. Since then, various blockchain sharding solutions have been proposed for blockchain capacity expansion,making sharding a mainstream blockchain expansion technology. Sharding helps not only to avoid erosion of the extent of decentralization within the system, but also supports the expansion of blockchain storage capacity and reduces the severity of scalability problems faced by blockchain applications [11]. Sharding divides the complete blockchain network into multiple separate fragments, each of which is maintained by a defined group of network nodes. All transaction processing and the storage of state data are completed within the fragment [12]. Multiple transactions generated on the blockchain, through parallel processing within each fragment, enable approximately linear improvement of the transaction processing speed. Meanwhile, transaction data are stored in different fragments to reduce the pressure of single-node storage. Here we propose a DHT-based blockchain dual-sharding storage extension mechanism--DBDSM--that uses sharding to achieve scalable storage capacity within the blockchain. DBDSM has been designed to enable nodes to realize their basic functions and simultaneously reduce data storage requirements. A mixed query mechanism for transaction data in the fragmented state is deployed to improve the efficiency of the transaction data queries. The main focus of the DBDSM is to improve blockchain storage expansion capabilities. The main contributions of this paper are as follows:  
1. We designed a DHT-based blockchain dual-sharding storage extension mechanism--DBDSM. The transaction data of the whole network are processed by the second fragmentation and stored in nodes across different clusters, providing security and availability of data while reducing transaction data storage requirements at individual nodes, thus realizing the expansion of the blockchain storage capacity. 2. A hybrid query mechanism for transaction data in the fragmented state is proposed.