Copy from S3 to RDS Restore Instance Limited @ 100MB/s Regardless of Instance & Storage Limits

The issue you're experiencing with the 100MB/s limit during the RDS restore process from an S3 backup is not explicitly mentioned in the provided sources. However, based on the information available, there are a few points to consider:

1. RDS integration with S3 has some inherent limitations. While these don't directly mention a 100MB/s transfer limit, they do indicate that there are constraints on how RDS interacts with S3.

2. The restore process you're using might be different from the native backup and restore functionality that's available for certain RDS instances, like RDS Custom for SQL Server 2022. This native functionality allows for more direct interaction with S3 buckets.

3. The consistent 100MB/s limit across different instance types (t3 or r5) suggests that this might be a service-level limitation rather than an instance-specific one.

4. The fact that you can transfer the file to AWS over the public internet faster than the internal S3 to RDS copy is unusual, as typically internal AWS network transfers are faster than external ones.

Given these points, here are some suggestions:

1. Check if your RDS instance type supports native backup and restore with S3 integration. If it does, this might provide better performance.

2. Consider splitting your large 300+GB file into smaller chunks. Some RDS features have file size limitations (e.g., 50GB for uploads from RDS to S3).

3. Reach out to AWS Support for clarification on any specific limits for the RDS restore from backup process, as this might be a service-specific limitation not publicly documented.

4. Explore alternative backup and restore methods that might offer better performance, such as using AWS Database Migration Service (DMS) or other AWS-native tools designed for large data transfers.

Remember that while these suggestions might help, the specific 100MB/s limit you're encountering isn't directly addressed in the available information, and might require further investigation with AWS support.
Sources
Native backup and restore with Amazon S3 integration on Amazon RDS Custom for SQL Server 2022 | AWS Database Blog
Integrating an Amazon RDS for SQL Server DB instance with Amazon S3 - Amazon Relational Database Service