By design, storage scaling operations for an Amazon RDS instance have a minimal impact on ongoing database operations. For this reason, the operations are long running and can take time to complete. The following factors can affect the time required to increase the allocated storage of an Amazon RDS DB instance:

• If you move between storage media types, such as from magnetic to SSD or PIOPS, downtime is required at the beginning of the process to enable or disable any EBS optimizations on the destination media type. If an instance uses a single Availability Zone, the downtime can be several minutes. If an instance uses multiple Availability Zones, less downtime is required.
• If you have large workload running at the time of the operation, the operation can take longer to complete because the time required to complete a storage operation is proportional to the running workload. It's a best practice to schedule storage increase operations outside peak hours to reduce the time required to complete the operation.
• If you have high-load conditions, create a read replica for an RDS database instance, perform storage scaling operations on the read replica, and then promote the read replica instance to the primary instance.

After a storage modification has started, the operation can't be canceled, and the instance status is modifying until the Amazon Elastic Block Store (Amazon EBS) operations are complete. You can Restore a DB Instance to a Specified Time or Restore from a DB Snapshot to create a new instance with the original storage configuration. An instance that's restored won't be in the modifying status.