I am looking to increase storage of two RDS instances (just the storage space allocated, not the instance type or other parameters). The documentation at https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PIOPS.StorageTypes.html#USER_PIOPS.ModifyingExisting suggests:
You can change from standard storage to Provisioned IOPS storage, or from Provisioned IOPS to standard storage, as well as increase storage, with little to no downtime.
I would definitely schedule a maintenance window before performing the change. But the documentation seems a little vague in this area. For someone who might have done this before, what is "little to no downtime"? Can I expect 5 seconds or is it more like 5 minutes?
Update July, 2019:
I've updated the link to the correct and updated AWS documentation (which was broken). The newer documentation has a blurb that helps answer the original question as well:
In most cases, scaling storage doesn't require any outage and doesn't degrade performance of the server. After you modify the storage size for a DB instance, the status of the DB instance is Storage-optimization. The DB instance is fully operational after a storage modification. However, you can't make further storage modifications either for six hours or while the DB instance status is storage-optimization, whichever is longer.
However, a special case is if you have a SQL Server DB instance and haven't modified the storage configuration since November 2017. In this case, you might experience a short outage of a few minutes when you modify your DB instance to increase the allocated storage. After the outage, the DB instance is online but in the Storage-optimization state. Performance might be degraded during storage optimization.