What is the best way to increase the storage space for RDS without any downtime

Hi There

See https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIOPS.StorageTypes.html#USER_PIOPS.ModifyingExisting

If you need space for additional data, you can scale up the storage of an existing DB instance. To do so, you can use the Amazon RDS Management Console, the Amazon RDS API, or the AWS Command Line Interface (AWS CLI).

Scaling storage usually doesn't cause any outage or performance degradation of the DB instance. After you modify the storage size for a DB instance, the status of the DB instance is storage-optimization.