On-Demand Instances let you pay for database migration capacity by the hour with no long-term commitments. This frees you from the costs and complexities of planning, purchasing, and maintaining hardware, thus making the entire process quick and inexpensive. AWS Database Migration Service also supports continuous data replication. You have the option of enabling Multi-AZ which provides a replication stream that is fault tolerant through redundant replication servers.

AWS Database Migration Service currently supports the T2, C4 and R4 instance classes (R4 in limited regions). T2 instances are low-cost standard instances designed to provide a baseline level of CPU performance with the ability to burst above the baseline. They are suitable for developing, configuring and testing your database migration process, and for periodic data migration tasks that can benefit from the CPU burst capability. C4 and R4 instances are designed to deliver the highest level of processor performance and achieve significantly higher packet per second (PPS) performance, lower network jitter, and lower network latency. You should use C4 or R4 instances if you are migrating large databases and are looking to minimize the migration time.