AmazonRDS has exceeded 85% of the usage limit

Your database instance storage isn't just for your data, it's used by the underlying engine and logs etc. So there's going to be a block of it used up that's fairly static overhead that doesn't scale up with your data.

If you're considering a self-managed DB instead of RDS make sure you consider the total cost of ownership. Our experience is RDS has saved us a bundle because there's so much less maintenance.

Unless using Aurora Serverless, you'll be charged for the time your instance is running as per the pricing page. Doesn't matter if the instance is idle or has no connections, it's still running. You can temporarily stop an instance for up to 7 days and you will be charged only for the EBS storage. Or you can take a backup and terminate it, restoring it later, to save even more.

If your DB is going to have widely varying usage patterns Aurora Serverless is worth a look.

In addition to what has already been mentioned by skinsman, there are some other objects that can take space in the database. For example, temporary table space is one of them. Please ask customer to run the following queries to find out space utilization by database and temporary tablespace. If the size of temporary tablespace is high, please ask customer to restart the database instance, which will release the space.

For Data Files: SELECT file_name, tablespace_name, table_name, engine, index_length, total_extents, extent_size from information_schema.files WHERE file_name LIKE '%ibdata%';

For temporary tablespaces: SELECT file_name, tablespace_name, table_name, engine, index_length, total_extents, extent_size from information_schema.files WHERE file_name LIKE '%ibtmp%';