DMS instance sizing guidance

Kindly note DMS using replication instance i.e. a middleware between source and target database and it doesn't necessarily need the same amount of storage and resources on it. Even source data isn't stored on replication instance except for changes happened during full load load or if dms isn't processing the changes to target with the same speed they are being pushed on source.

It is not recommended to use t2 instance type due to their burst balance capability which won't be sufficient for production migration. Then we have c4 instance type that are network optimized and would be beneficial when DMS has to transfer archive logs to replication for e.g. that happens in case of using Oracle Binary reader option.

Also there are basic guidelines regarding choosing the replication instance in our public documentation. This highlights that "When you plug heavy workloads with a high number of transactions per second (TPS) into this architecture, you can find the extra memory provided by R4 instances useful. You can use R4 instances to hold a large number of transactions in memory and prevent memory-pressure issues during ongoing replications."

Even once you select a replication instance and later you think you want to resize, you can do it while tasks are running. In the backend tasks will just stop and resume automatically once the scale compute of replication instance is complete. Or you can use the blog to use solution to dynamically scale dms replication instance.