## 2 Blockchain-Based Data Storage Methods: On-Chain, Off-Chain, and Hybrid Approaches
### Hybrid Storage Approaches  
Hybrid storage solutions, which integrate on-chain and off-chain approaches, provide a balanced trade-off between security and performance. In this model, small and critical data are stored on the blockchain, while large datasets are maintained in external storage solutions. This approach reduces costs and improves system performance, while preserving the security advantages of blockchain [89,90]. By storing large datasets off-chain and keeping only verification hash values on-chain, hybrid storage optimizes cost efficiency and system scalability [91]. In sectors like healthcare, hybrid solutions help preserve data privacy while reducing costs and enhancing performance. For instance, large-scale electronic health records can be securely stored in off-chain storage infrastructures [13].  
Hybrid storage combines blockchain and off-chain systems, balancing security, scalability, and cost [16,37]. These methods retain blockchain's immutability and security while leveraging the flexibility and expanded storage capacity of external systems [46,48]. Hybrid storage works by storing a portion of the data on-chain (e.g., references or cryptographic hashes) and the remaining data in external platforms (e.g., cloud storage or P2P networks) [15,47]. This approach is particularly effective for managing large datasets and reducing storage costs. By integrating the security of decentralized networks with the high scalability of external storage solutions, hybrid storage creates a versatile framework for various industries [42,43]. This model enhances data integrity, accessibility, and performance, making it an efficient and adaptable storage solution for blockchain-based applications. Table 3 presents the classification of hybrid data storage solutions for blockchain technologies.  
\begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Data Storage Environment** & **Storage Mechanism** & **Implementation Strategy** & **Advantages** & **Disadvantages** & **Examples** \\ \hline Blockchain + Off-chain Integrations & IPFS CIDs [15,92] & Reference information on the blockchain for off-chain data & Verifiability of data & The data itself is not stored on the blockchain & IPFS integrations \\ \hline Distributed and Decentralized Cloud Solutions & Hybrid Cloud + Blockchain [42,93] & Integrated use of cloud infrastructures and blockchain & High infrastructures and blockchain low cost & Security may scalability, low cost & Ankr, Ocean depend on external providers & Ankr, Ocean Protocol \\ \hline \multirow{2}{*}{Cloud Solutions} & Lightning Network [94] & Channel-based transactions, results integration with blockchain & Fast transactions, low costs. & Only suitable for micro transactions & Bitcoin Lightning Network \\ \hline \multirow{6}{*}{Layer-2 Optimizations} & Rollups [19,20] & Processing transactions at Layer-2 and saving summary data at Layer-1 & High scalability & Technical complexity & Arbitrum, zksync \\ \cline{2-5}  & Plasma [52,95] & Processing data on subchains and recording digests to the main blockchain & A reliable Layer-2 solution & Coordination between sub-chains can be difficult & OMG Network \\ \cline{1-1} \cline{2-5}  & State Channels [96,97] & Off-chain transactions are on the summary blockchain only & Very low latency & Complicated installation & Lightning Network \\ \hline \hline \end{tabular}
\end{table}
Table 3: This table categorizes hybrid storage solutions combining blockchain with off-chain methods, decentralized cloud, and Layer-2 scaling. These approaches optimize security, cost, and scalability through on-chain and off-chain strategies, leveraging IPFS CIDs, hybrid clouds, rollups, and state channels for efficient data management.