**1. Introduction**

Blockchain systems are categorized into permissionless blockchains (a.k.a. a public blockchain) and permissioned blockchains (a.k.a. a consortium/private blockchain). In a permissionless blockchain system, any computer or user that can access the blockchain network can join it or quit at will. The nodes and users of the permissionless blockchain networks are identified by their public keys. Cryptocurrencies such as Bitcoin [1], Ethereum [2], and EOS [3] are constructed as permissionless blockchain systems. Permissioned blockchains are always used for information-sharing among several stakeholders. The nodes and users that want to join the network need to be authorized. Certification Authority (CA) can be used to perform the authorization. HLF [4] and Quorum [5] are typical permissioned blockchain systems.

Blockchain provides a tamper-proof distributed ledger by mechanisms such as decentralized architecture, consensus algorithm, asymmetric encryption, and so on. Transactions that are sent to the blockchain system are packed into blocks by certain rules. A block contains a block header and a block body, the transactions are recorded in the block body. Each block contains the hash of its previous block, so that the blocks form a chain structure, which means that tampering with a historical transaction can be obtained by comparing the block hash saved in the next block. The blocks are usually stored as files in the file system of blockchain nodes. Typically, each node of the blockchain network retains a full copy of the entire chain of blocks, so that each node can check the integrity of the data locally.

Since the size of the chain of blocks increases continually, the blockchain systems are facing a storage scalability issue. For permissionless blockchains, the data size of a

**Citation:** Sun, H.; Pi, B.; Sun, J.; Miyamae, T.; Morinaga, M. SASLedger: A Secured, Accelerated Scalable Storage Solution for Distributed Ledger Systems. *Future Internet* **2021**, *13*, 310. https:// doi.org/10.3390/fi13120310

Academic Editors: Ahad Zare Ravasan, Taha Mansouri, Michal Krˇcál and Saeed Rouhani

Received: 5 November 2021 Accepted: 29 November 2021 Published: 30 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Bitcoin node has reached 366.51 GB [6] and the data size of a Ethereum node has reached 987.54 GB [7] by 28 September 2021; for permissioned blockchains, the issue has also caused concern [8,9]. The storage burden is becoming more onerous for individual participants who run blockchain nodes with their personal computers. This situation may injure the decentralized character of blockchain systems since only wealthy individuals or organizations can afford the increasing storage scaling demand.

Methods proposed to solve the storage scalability issue of blockchain systems are divided into "on-chain" solutions and "off-chain" solutions. On-chain solutions reduce the contents stored in each node by altering the range of consensus or the contents of transactions [10–13]. Off-chain solutions provide off-chain storage devices to store data, but most of them focus on reducing the data size before the data are uploaded to blockchains [14–17]. This approach uses blockchain as a proof repository to ensure the integrity of off-chain data, but the off-chain data cannot be retrieved through blockchain node, on the contrary, block archiver [18] reduces the data size of the data that has been uploaded to the blockchain and stored in the blockchain nodes, but block archiver fails to ensure the integrity of the data stored off-chain, which severely damages the tamper-proof property of the original blockchain system. To bridge this gap, an off-chain solution for the data that has been uploaded to the blockchain with tamper-proof property is proposed in this paper. The data are eliminated from the blockchain node and transmitted to a remote DB server while the node saves a concise vector commitment (VC) [19] to ensure the integrity of the data stored off-chain.

The contributions made by the present research are as follows:


The rest of the paper is organized as follows: Section 2 describes different approaches used when solving the scalability issues of blockchain. Section 3 introduces the transactionprocessing model and storage structure of the HLF blockchain system and the mechanism of VC. Section 4 introduces our solution and then analyzes the security and performance features thereof. Section 5 demonstrates the details of our implementation based on HLF. Section 6 shows our experimental results and their evaluations. Section 7 concludes with recommendations for future research.
