If you expect an increase in your data load (such as a load that is larger than 20% of your RDS DB instance's current storage size), it's a best practice to manually increase the size of your RDS DB storage before the data load. For example, if your RDS DB instance's storage size is 100 GiB and has 10.1 GiB of free space, and the storage consumption is usually 100 MB per hour, then autoscaling will increase the storage by 10% (10 GiB).
But if you expect to load 50 GiB of data in less than one hour, the 10% storage increase won't be sufficient. And because the previous autoscaling operation is in the storage modification cooldown period, the RDS DB instance can't have any storage modifications for at least six hours, resulting in an RDS DB instance that is in a storage-full status for several hours.
For more information just visit here https://aws.amazon.com/premiumsupport/knowledge-center/rds-autoscaling-low-free-storage/