  The fast developing Industrial Internet of Things (IIoT) technologies provide
a promising opportunity to build large-scale systems to connect numerous
heterogeneous devices into the Internet. Most existing IIoT infrastructures are
based on a centralized architecture, which is easier for management but cannot
effectively support immutable and verifiable services among multiple parties.
Blockchain technology provides many desired features for large-scale IIoT
infrastructures, such as decentralization, trustworthiness, trackability, and
immutability. This paper presents a blockchain-based IIoT architecture to
support immutable and verifiable services. However, when applying blockchain
technology to the IIoT infrastructure, the required storage space posts a grant
challenge to resource-constrained IIoT infrastructures. To address the storage
issue, this paper proposes a hierarchical blockchain storage structure,
\textit{ChainSplitter}. Specially, the proposed architecture features a
hierarchical storage structure where the majority of the blockchain is stored
in the clouds, while the most recent blocks are stored in the overlay network
of the individual IIoT networks. The proposed architecture seamlessly binds
local IIoT networks, the blockchain overlay network, and the cloud
infrastructure together through two connectors, the \textit{blockchain
connector} and the \textit{cloud connector}, to construct the hierarchical
blockchain storage. The blockchain connector in the overlay network builds
blocks in blockchain from data generated in IIoT networks, and the cloud
connector resolves the blockchain synchronization issues between the overlay
network and the clouds. We also provide a case study to show the efficiency of
the proposed hierarchical blockchain storage in a practical Industrial IoT
case.
