How to reduce storage(scale down) my RDS instance?

RDS does not allow you to reduce the amount of storage allocated to a database instance, only increase it.

To move your database to less storage you would have to create a new RDS instance with your desired storage space, then use something like pg_dump/pg_restore to move the data from the old database to the new one.

Also be aware that an RDS instance with 1,000GB of SSD storage has a base IOPS of 3,000. An RDS instance with 100GB of SSD storage has a base IOPS of 300, with occasional bursts of up to 3,000.

Based on AWS's help here, this is the full process that worked for me:

1) Dump the database to a file: run this on a machine that has network access to the database: pg_dump -Fc -v -h your-rds-endpoint.us-west-2.rds.amazonaws.com -U your-username your-databasename > your-databasename.dump

2) In the AWS console, create a new RDS instance with smaller storage. (You probably want to set it up with the same username, password, and database name.)

3) Restore the database on the new RDS instance: run this command (obviously on the same machine as the previous command): pg_restore -v -h the-new-rds-endpoint.us-west-2.rds.amazonaws.com -U your-username -d your-databasename your-databasename.dump

(Note, in step 3, that I'm using the endpoint of the new RDS instance. Also note there's no :5432 at the end of the endpoint addresses.)

Amazon doesn't allow to reduce size of HDD of RDS instance, you may have two options to reduce size of storage.

1:-if you can afford downtimes, then mysqldump backup of old instance can be restored to new instance having lesser storage size.

2:- You can use Database migration service to move data from one instance to another instance without any downtime.