  Service Level Agreements (SLA) are employed to ensure the performance of
Cloud solutions. When a component fails, the importance of logs increases
significantly. All departments may turn to logs to determine the cause of the
issue and find the party at fault. The party at fault may be motivated to
tamper with the logs to hide their role. We argue that the critical nature of
Cloud logs calls for immutability and verification mechanism without the
presence of a single trusted party.
  This paper proposes such a mechanism by describing a blockchain-based log
storage system, called Logchain, which can be integrated with existing private
and public blockchain solutions. Logchain uses the immutability feature of
blockchain to provide a tamper-resistance platform for log storage.
Additionally, we propose a hierarchical structure to address blockchains'
scalability issues. To validate the mechanism, we integrate Logchain into
Ethereum and IBM Blockchain. We show that the solution is scalable and perform
the analysis of the cost of ownership to help a reader select an implementation
that would address their needs.
  The Logchain's scalability improvement on a blockchain is achieved without
any alteration of blockchains' fundamental architecture. As shown in this work,
it can function on private and public blockchains and, therefore, can be a
suitable alternative for organizations that need a secure, immutable log
storage platform.
